docs/core-api: memory-allocation: describe reclaim behaviour

docs/core-api: memory-allocation: describe reclaim behaviour

Changelog of commit dcda9b04713c ("mm, tree wide: replace __GFP_REPEAT by
__GFP_RETRY_MAYFAIL with more useful semantic") has very nice description
of GFP flags that affect reclaim behaviour of the page allocator.

It would be pity to keep this description buried in the log so let's expose
it in the Documentation/ as well.

Cc: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20200719153641.231131-1-rppt@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>