docs: add VmPMD description in proc

commit dc6c9a35 ("mm: account pmd page tables to the process")
add VmPMD in /proc/PID/status.
This patch add a description in proc.txt for it.

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>