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>

Documentation/core-api/mm-api.rst

@@ -49,8 +49,14 @@ The Slab Cache
 .. kernel-doc:: mm/util.c
    :functions: kfree_const kvmalloc_node kvfree
 
-More Memory Management Functions
-================================
+Virtually Contiguous Mappings
+=============================
+
+.. kernel-doc:: mm/vmalloc.c
+   :export:
+
+File Mapping and Page Cache
+===========================
 
 .. kernel-doc:: mm/readahead.c
    :export:
@@ -58,23 +64,28 @@ More Memory Management Functions
 .. kernel-doc:: mm/filemap.c
    :export:
 
-.. kernel-doc:: mm/memory.c
+.. kernel-doc:: mm/page-writeback.c
    :export:
 
-.. kernel-doc:: mm/vmalloc.c
+.. kernel-doc:: mm/truncate.c
    :export:
 
-.. kernel-doc:: mm/page_alloc.c
-   :internal:
+Memory pools
+============
 
 .. kernel-doc:: mm/mempool.c
    :export:
 
+DMA pools
+=========
+
 .. kernel-doc:: mm/dmapool.c
    :export:
 
-.. kernel-doc:: mm/page-writeback.c
-   :export:
+More Memory Management Functions
+================================
 
-.. kernel-doc:: mm/truncate.c
+.. kernel-doc:: mm/memory.c
    :export:
+
+.. kernel-doc:: mm/page_alloc.c