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>