doc: Clarify that nmi_watchdog param is for hardlockups

The kernel NMI watchdog acts as both a hardlockup and softlockup detector.
However, the kernel parameter nmi_watchdog can only enable or disable the
hardlockup detector. Clarify that in the documentation.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>