Documentation/filesystems/proc.txt: add more description for maps/smaps

Add some more description on the limitations for smaps/maps readings, as
well as some guaruntees we can make.

Link: http://lkml.kernel.org/r/1475296958-27652-2-git-send-email-robert.hu@intel.com


Signed-off-by: Robert Ho <robert.hu@intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: Robert Hu <robert.hu@intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>