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-fwrite.rst

@@ -0,0 +1,79 @@
+.. 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_fwrite:
+
+========================
+Digital TV demux write()
+========================
+
+Name
+----
+
+Digital TV demux write()
+
+
+Synopsis
+--------
+
+.. c:function:: ssize_t write(int fd, const void *buf, size_t count)
+    :name: dvb-dmx-write
+
+Arguments
+---------
+
+``fd``
+  File descriptor returned by a previous call to :c:func:`open() <dvb-ca-open>`.
+
+``buf``
+     Buffer with data to be written
+
+``count``
+    Number of bytes at the buffer
+
+Description
+-----------
+
+This system call is only provided by the logical device
+``/dev/dvb/adapter?/dvr?``, associated with the physical demux device that
+provides the actual DVR functionality. It is used for replay of a
+digitally recorded Transport Stream. Matching filters have to be defined
+in the corresponding physical demux device, ``/dev/dvb/adapter?/demux?``.
+The amount of data to be transferred is implied by count.
+
+
+Return Value
+------------
+
+On success 0 is returned.
+
+On error -1 is returned, and the ``errno`` variable is set
+appropriately.
+
+.. tabularcolumns:: |p{2.5cm}|p{15.0cm}|
+
+.. flat-table::
+    :header-rows:  0
+    :stub-columns: 0
+    :widths: 1 16
+
+    -  -  ``EWOULDBLOCK``
+       -  No data was written. This might happen if ``O_NONBLOCK`` was
+	  specified and there is no more buffer space available (if
+	  ``O_NONBLOCK`` is not specified the function will block until buffer
+	  space is available).
+
+    -  -  ``EBUSY``
+       -  This error code indicates that there are conflicting requests. The
+	  corresponding demux device is setup to receive data from the
+	  front- end. Make sure that these filters are stopped and that the
+	  filters with input set to ``DMX_IN_DVR`` are started.
+
+The generic error codes are described at the
+:ref:`Generic Error Codes <gen-errors>` chapter.