Add references to GFP documentation and the memory-allocation.rst and remove
GFP_USER, GFP_DMA and GFP_NOIO descriptions.

While on it slightly change the formatting so that the list of GFP flags
will be rendered as "description" in the generated html.
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>