Documentation: vm: Add 1GB large page support information

This patch adds 1GB large page support information in
Documentation/vm/hugetlbpage.txt

Reference:
https://lkml.org/lkml/2014/10/31/366



Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>