Docs: Document that the sticky bit is understood by hugetlbfs

Commit 75897d60

 (hugetlb: allow sticky directory mount option) added
support for mounting hugetlbfs with sticky option set, like /tmp is
usually mounted, but forgot to document that.

Acked-by: Ken Chen <kenchen@google.com>
Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>