partitions/efi: complete documentation of gpt kernel param purpose

The usage of the 'gpt' kernel parameter is twofold: (i) skip any mbr
integrity checks and (ii) enable the backup GPT header to be used in
situations where the primary one is corrupted.  This last "feature" is not
obvious and needs to be properly documented in the kernel-parameters
document.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=63591



Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Matt Domsch <Matt_Domsch@dell.com>
Cc: Matt Fleming <matt.fleming@intel.com>
Cc: "Chandramouleeswaran,Aswin" <aswin@hp.com>
Cc: Chris Murphy <bugzilla@colorremedies.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -1043,7 +1043,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			debugfs files are removed at module unload time.
 
 	gpt		[EFI] Forces disk with valid GPT signature but
-			invalid Protective MBR to be treated as GPT.
+			invalid Protective MBR to be treated as GPT. If the
+			primary GPT is corrupted, it enables the backup/alternate
+			GPT to be used instead.
 
 	grcan.enable0=	[HW] Configuration of physical interface 0. Determines
 			the "Enable 0" bit of the configuration register.