Documentation/kernel-parameters.txt: update mem= option's spec according to its implementation

Current mem= implementation seems buggy because the specification and
implementation don't match.  The current mem= has been working for many
years and it's not buggy - it works as expected.  So we should update the
specification.
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -1503,9 +1503,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	mem=nn[KMG]	[KNL,BOOT] Force usage of a specific amount of memory
 			Amount of memory to be used when the kernel is not able
 			to see the whole system memory or for test.
-			[X86-32] Use together with memmap= to avoid physical
-			address space collisions. Without memmap= PCI devices
-			could be placed at addresses belonging to unused RAM.
+			[X86] Work as limiting max address. Use together
+			with memmap= to avoid physical address space collisions.
+			Without memmap= PCI devices could be placed at addresses
+			belonging to unused RAM.
 
 	mem=nopentium	[BUGS=X86-32] Disable usage of 4MB pages for kernel
 			memory.