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/dvb-fe-read-status.rst

@@ -0,0 +1,32 @@
+.. 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
+
+.. _dvb-fe-read-status:
+
+***************************************
+Querying frontend status and statistics
+***************************************
+
+Once :ref:`FE_SET_PROPERTY <FE_GET_PROPERTY>` is called, the
+frontend will run a kernel thread that will periodically check for the
+tuner lock status and provide statistics about the quality of the
+signal.
+
+The information about the frontend tuner locking status can be queried
+using :ref:`FE_READ_STATUS`.
+
+Signal statistics are provided via
+:ref:`FE_GET_PROPERTY`.
+
+.. note::
+
+   Most statistics require the demodulator to be fully locked
+   (e. g. with :c:type:`FE_HAS_LOCK <fe_status>` bit set). See
+   :ref:`Frontend statistics indicators <frontend-stat-properties>` for
+   more details.