media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/dvb/dmx-munmap.rst

@@ -0,0 +1,63 @@
+.. Permission is granted to copy, distribute and/or modify this
+.. document under the terms of the GNU Free Documentation License,
+.. Version 1.1 or any later version published by the Free Software
+.. Foundation, with no Invariant Sections, no Front-Cover Texts
+.. and no Back-Cover Texts. A copy of the license is included at
+.. Documentation/userspace-api/media/fdl-appendix.rst.
+..
+.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _dmx-munmap:
+
+************
+DVB munmap()
+************
+
+Name
+====
+
+dmx-munmap - Unmap device memory
+
+.. warning:: This API is still experimental.
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <unistd.h>
+    #include <sys/mman.h>
+
+
+.. c:function:: int munmap( void *start, size_t length )
+    :name: dmx-munmap
+
+Arguments
+=========
+
+``start``
+    Address of the mapped buffer as returned by the
+    :ref:`mmap() <dmx-mmap>` function.
+
+``length``
+    Length of the mapped buffer. This must be the same value as given to
+    :ref:`mmap() <dmx-mmap>`.
+
+
+Description
+===========
+
+Unmaps a previously with the :ref:`mmap() <dmx-mmap>` function mapped
+buffer and frees it, if possible.
+
+
+Return Value
+============
+
+On success :ref:`munmap() <dmx-munmap>` returns 0, on failure -1 and the
+``errno`` variable is set appropriately:
+
+EINVAL
+    The ``start`` or ``length`` is incorrect, or no buffers have been
+    mapped yet.