PM: docs: Delete the obsolete states.txt document

The Documentation/power/states.txt document is now redundant and
sonewhat outdated, so delete it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/power/states.txt

-System Power Management Sleep States
-
-(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
-
-The kernel supports up to four system sleep states generically, although three
-of them depend on the platform support code to implement the low-level details
-for each state.
-
-The states are represented by strings that can be read or written to the
-/sys/power/state file.  Those strings may be "mem", "standby", "freeze" and
-"disk", where the last three always represent Power-On Suspend (if supported),
-Suspend-To-Idle and hibernation (Suspend-To-Disk), respectively.
-
-The meaning of the "mem" string is controlled by the /sys/power/mem_sleep file.
-It contains strings representing the available modes of system suspend that may
-be triggered by writing "mem" to /sys/power/state.  These modes are "s2idle"
-(Suspend-To-Idle), "shallow" (Power-On Suspend) and "deep" (Suspend-To-RAM).
-The "s2idle" mode is always available, while the other ones are only available
-if supported by the platform (if not supported, the strings representing them
-are not present in /sys/power/mem_sleep).  The string representing the suspend
-mode to be used subsequently is enclosed in square brackets.  Writing one of
-the other strings present in /sys/power/mem_sleep to it causes the suspend mode
-to be used subsequently to change to the one represented by that string.
-
-Consequently, there are two ways to cause the system to go into the
-Suspend-To-Idle sleep state.  The first one is to write "freeze" directly to
-/sys/power/state.  The second one is to write "s2idle" to /sys/power/mem_sleep
-and then to write "mem" to /sys/power/state.  Similarly, there are two ways
-to cause the system to go into the Power-On Suspend sleep state (the strings to
-write to the control files in that case are "standby" or "shallow" and "mem",
-respectively) if that state is supported by the platform.  In turn, there is
-only one way to cause the system to go into the Suspend-To-RAM state (write
-"deep" into /sys/power/mem_sleep and "mem" into /sys/power/state).
-
-The default suspend mode (ie. the one to be used without writing anything into
-/sys/power/mem_sleep) is either "deep" (if Suspend-To-RAM is supported) or
-"s2idle", but it can be overridden by the value of the "mem_sleep_default"
-parameter in the kernel command line.  On some ACPI-based systems, depending on
-the information in the FADT, the default may be "s2idle" even if Suspend-To-RAM
-is supported.
-
-The properties of all of the sleep states are described below.
-
-
-State:		Suspend-To-Idle
-ACPI state:	S0
-Label:		"s2idle" ("freeze")
-
-This state is a generic, pure software, light-weight, system sleep state.
-It allows more energy to be saved relative to runtime idle by freezing user
-space and putting all I/O devices into low-power states (possibly
-lower-power than available at run time), such that the processors can
-spend more time in their idle states.
-
-This state can be used for platforms without Power-On Suspend/Suspend-to-RAM
-support, or it can be used in addition to Suspend-to-RAM to provide reduced
-resume latency.  It is always supported.
-
-
-State:		Standby / Power-On Suspend
-ACPI State:	S1
-Label:		"shallow" ("standby")
-
-This state, if supported, offers moderate, though real, power savings, while
-providing a relatively low-latency transition back to a working system.  No
-operating state is lost (the CPU retains power), so the system easily starts up
-again where it left off. 
-
-In addition to freezing user space and putting all I/O devices into low-power
-states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline
-and all low-level system functions are suspended during transitions into this
-state.  For this reason, it should allow more energy to be saved relative to
-Suspend-To-Idle, but the resume latency will generally be greater than for that
-state.
-
-
-State:		Suspend-to-RAM
-ACPI State:	S3
-Label:		"deep"
-
-This state, if supported, offers significant power savings as everything in the
-system is put into a low-power state, except for memory, which should be placed
-into the self-refresh mode to retain its contents.  All of the steps carried out
-when entering Power-On Suspend are also carried out during transitions to STR.
-Additional operations may take place depending on the platform capabilities.  In
-particular, on ACPI systems the kernel passes control to the BIOS (platform
-firmware) as the last step during STR transitions and that usually results in
-powering down some more low-level components that aren't directly controlled by
-the kernel.
-
-System and device state is saved and kept in memory.  All devices are suspended
-and put into low-power states.  In many cases, all peripheral buses lose power
-when entering STR, so devices must be able to handle the transition back to the
-"on" state.
-
-For at least ACPI, STR requires some minimal boot-strapping code to resume the
-system from it.  This may be the case on other platforms too.
-
-
-State:		Suspend-to-disk
-ACPI State:	S4
-Label:		"disk"
-
-This state offers the greatest power savings, and can be used even in
-the absence of low-level platform support for power management. This
-state operates similarly to Suspend-to-RAM, but includes a final step
-of writing memory contents to disk. On resume, this is read and memory
-is restored to its pre-suspend state. 
-
-STD can be handled by the firmware or the kernel. If it is handled by
-the firmware, it usually requires a dedicated partition that must be
-setup via another operating system for it to use. Despite the
-inconvenience, this method requires minimal work by the kernel, since
-the firmware will also handle restoring memory contents on resume. 
-
-For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used
-to write memory contents to free swap space. swsusp has some restrictive
-requirements, but should work in most cases. Some, albeit outdated,
-documentation can be found in Documentation/power/swsusp.txt.
-Alternatively, userspace can do most of the actual suspend to disk work,
-see userland-swsusp.txt.
-
-Once memory state is written to disk, the system may either enter a
-low-power state (like ACPI S4), or it may simply power down. Powering
-down offers greater savings, and allows this mechanism to work on any
-system. However, entering a real low-power state allows the user to
-trigger wake up events (e.g. pressing a key or opening a laptop lid).