Documentation: update hugetlb information

The hugetlb documentation has gotten a bit out of sync with the current code.
Updated the sysctl file to refer to Documentation/vm/hugetlbpage.txt.  Update
that file to contain the current state of affairs (with the newer named sysctl
in place).

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Acked-by: Adam Litke <agl@us.ibm.com>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>