docs/vm: page_frags convert to ReST format

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vm/page_frags

+.. _page_frags:
+
+==============
 Page fragments
---------------
+==============
 
 A page fragment is an arbitrary-length arbitrary-offset area of memory
 which resides within a 0 or higher order compound page.  Multiple