docs/core-api: make mm-api.rst more structured

docs/core-api: make mm-api.rst more structured

The mm-api.rst covers variety of memory management APIs under "More Memory
Management Functions" section. The descriptions included there are in a
random order there are quite a few of them which makes the section too
long.

Regrouping the documentation by subject and splitting the long "More Memory
Management Functions" section into several smaller sections makes the
generated html more usable.

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