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/v4l/dev-radio.rst

@@ -0,0 +1,59 @@
+.. 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
+
+.. _radio:
+
+***************
+Radio Interface
+***************
+
+This interface is intended for AM and FM (analog) radio receivers and
+transmitters.
+
+Conventionally V4L2 radio devices are accessed through character device
+special files named ``/dev/radio`` and ``/dev/radio0`` to
+``/dev/radio63`` with major number 81 and minor numbers 64 to 127.
+
+
+Querying Capabilities
+=====================
+
+Devices supporting the radio interface set the ``V4L2_CAP_RADIO`` and
+``V4L2_CAP_TUNER`` or ``V4L2_CAP_MODULATOR`` flag in the
+``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl. Other combinations of
+capability flags are reserved for future extensions.
+
+
+Supplemental Functions
+======================
+
+Radio devices can support :ref:`controls <control>`, and must support
+the :ref:`tuner or modulator <tuner>` ioctls.
+
+They do not support the video input or output, audio input or output,
+video standard, cropping and scaling, compression and streaming
+parameter, or overlay ioctls. All other ioctls and I/O methods are
+reserved for future extensions.
+
+
+Programming
+===========
+
+Radio devices may have a couple audio controls (as discussed in
+:ref:`control`) such as a volume control, possibly custom controls.
+Further all radio devices have one tuner or modulator (these are
+discussed in :ref:`tuner`) with index number zero to select the radio
+frequency and to determine if a monaural or FM stereo program is
+received/emitted. Drivers switch automatically between AM and FM
+depending on the selected frequency. The
+:ref:`VIDIOC_G_TUNER <VIDIOC_G_TUNER>` or
+:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` ioctl reports the
+supported frequency range.