docs/core-api: split memory management API to a separate file

This is basically copy-paste of the memory management section from
kernel-api.rst with some minor adjustments:

* The "User Space Memory Access" is moved to the beginning
* The get_user_pages_fast reference is now a part of "User Space Memory
  Access"
* And, of course, headings are adjusted with section being promoted to
  chapters

Link: http://lkml.kernel.org/r/1532626360-16650-6-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/index.rst

@@ -27,6 +27,7 @@ Core utilities
    errseq
    printk-formats
    circular-buffers
+   mm-api
    gfp_mask-from-fs-io
    timekeeping
    boot-time-mm

Documentation/core-api/kernel-api.rst

@@ -159,60 +159,6 @@ UUID/GUID
 .. kernel-doc:: lib/uuid.c
    :export:
 
-Memory Management in Linux
-==========================
-
-The Slab Cache
---------------
-
-.. kernel-doc:: include/linux/slab.h
-   :internal:
-
-.. kernel-doc:: mm/slab.c
-   :export:
-
-.. kernel-doc:: mm/util.c
-   :functions: kfree_const kvmalloc_node kvfree get_user_pages_fast
-
-User Space Memory Access
-------------------------
-
-.. kernel-doc:: arch/x86/include/asm/uaccess.h
-   :internal:
-
-.. kernel-doc:: arch/x86/lib/usercopy_32.c
-   :export:
-
-More Memory Management Functions
---------------------------------
-
-.. kernel-doc:: mm/readahead.c
-   :export:
-
-.. kernel-doc:: mm/filemap.c
-   :export:
-
-.. kernel-doc:: mm/memory.c
-   :export:
-
-.. kernel-doc:: mm/vmalloc.c
-   :export:
-
-.. kernel-doc:: mm/page_alloc.c
-   :internal:
-
-.. kernel-doc:: mm/mempool.c
-   :export:
-
-.. kernel-doc:: mm/dmapool.c
-   :export:
-
-.. kernel-doc:: mm/page-writeback.c
-   :export:
-
-.. kernel-doc:: mm/truncate.c
-   :export:
-
 Kernel IPC facilities
 =====================
 

Documentation/core-api/mm-api.rst

@@ -0,0 +1,57 @@
+======================
+Memory Management APIs
+======================
+
+User Space Memory Access
+========================
+
+.. kernel-doc:: arch/x86/include/asm/uaccess.h
+   :internal:
+
+.. kernel-doc:: arch/x86/lib/usercopy_32.c
+   :export:
+
+.. kernel-doc:: mm/util.c
+   :functions: get_user_pages_fast
+
+The Slab Cache
+==============
+
+.. kernel-doc:: include/linux/slab.h
+   :internal:
+
+.. kernel-doc:: mm/slab.c
+   :export:
+
+.. kernel-doc:: mm/util.c
+   :functions: kfree_const kvmalloc_node kvfree
+
+More Memory Management Functions
+================================
+
+.. kernel-doc:: mm/readahead.c
+   :export:
+
+.. kernel-doc:: mm/filemap.c
+   :export:
+
+.. kernel-doc:: mm/memory.c
+   :export:
+
+.. kernel-doc:: mm/vmalloc.c
+   :export:
+
+.. kernel-doc:: mm/page_alloc.c
+   :internal:
+
+.. kernel-doc:: mm/mempool.c
+   :export:
+
+.. kernel-doc:: mm/dmapool.c
+   :export:
+
+.. kernel-doc:: mm/page-writeback.c
+   :export:
+
+.. kernel-doc:: mm/truncate.c
+   :export: