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/cec/cec-func-open.rst

@@ -0,0 +1,85 @@
+.. 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
+
+.. _cec-func-open:
+
+**********
+cec open()
+**********
+
+Name
+====
+
+cec-open - Open a cec device
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <fcntl.h>
+
+
+.. c:function:: int open( const char *device_name, int flags )
+   :name: cec-open
+
+
+Arguments
+=========
+
+``device_name``
+    Device to be opened.
+
+``flags``
+    Open flags. Access mode must be ``O_RDWR``.
+
+    When the ``O_NONBLOCK`` flag is given, the
+    :ref:`CEC_RECEIVE <CEC_RECEIVE>` and :ref:`CEC_DQEVENT <CEC_DQEVENT>` ioctls
+    will return the ``EAGAIN`` error code when no message or event is available, and
+    ioctls :ref:`CEC_TRANSMIT <CEC_TRANSMIT>`,
+    :ref:`CEC_ADAP_S_PHYS_ADDR <CEC_ADAP_S_PHYS_ADDR>` and
+    :ref:`CEC_ADAP_S_LOG_ADDRS <CEC_ADAP_S_LOG_ADDRS>`
+    all return 0.
+
+    Other flags have no effect.
+
+
+Description
+===========
+
+To open a cec device applications call :c:func:`open() <cec-open>` with the
+desired device name. The function has no side effects; the device
+configuration remain unchanged.
+
+When the device is opened in read-only mode, attempts to modify its
+configuration will result in an error, and ``errno`` will be set to
+EBADF.
+
+
+Return Value
+============
+
+:c:func:`open() <cec-open>` returns the new file descriptor on success. On error,
+-1 is returned, and ``errno`` is set appropriately. Possible error codes
+include:
+
+``EACCES``
+    The requested access to the file is not allowed.
+
+``EMFILE``
+    The process already has the maximum number of files open.
+
+``ENFILE``
+    The system limit on the total number of open files has been reached.
+
+``ENOMEM``
+    Insufficient kernel memory was available.
+
+``ENXIO``
+    No device corresponding to this device special file exists.