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/2014/10/31/366Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/vm/hugetlbpage.txt

 
 The intent of this file is to give a brief summary of hugetlbpage support in
 the Linux kernel.  This support is built on top of multiple page size support
-that is provided by most modern architectures.  For example, i386
-architecture supports 4K and 4M (2M in PAE mode) page sizes, ia64
+that is provided by most modern architectures.  For example, x86 CPUs normally
+support 4K and 2M (1G if architecturally supported) page sizes, ia64
 architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
 256M and ppc64 supports 4K and 16M.  A TLB is a cache of virtual-to-physical
 translations.  Typically this is a very scarce resource on processor.