Documentation: correct overcommit caveat in hugetlbpage.txt

As shown by Gurudas Pai recently, we can put hugepages into the surplus
state (by echo 0 > /proc/sys/vm/nr_hugepages), even when
/proc/sys/vm/nr_overcommit_hugepages is 0. This is actually correct, to
allow the original goal (shrink the static pool to 0) to succeed (we are
converting hugepages to surplus because they are in use). However, the
documentation does not accurately reflect this case. Update it.

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Acked-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>