docs/mm: make GFP flags descriptions usable as kernel-doc

docs/mm: make GFP flags descriptions usable as kernel-doc

This patch adds DOC: headings for GFP flag descriptions and adjusts the
formatting to fit sphinx expectations of paragraphs.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Rapoport <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>