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>

Documentation/filesystems/proc.txt

@@ -235,6 +235,7 @@ Table 1-2: Contents of the status files (as of 4.1)
  VmExe                       size of text segment
  VmLib                       size of shared library code
  VmPTE                       size of page table entries
+ VmPMD                       size of second level page tables
  VmSwap                      size of swap usage (the number of referred swapents)
  Threads                     number of threads
  SigQ                        number of signals queued/max. number for queue