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/rc/lirc-set-measure-carrier-mode.rst

@@ -0,0 +1,53 @@
+.. 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
+
+.. _lirc_set_measure_carrier_mode:
+
+***********************************
+ioctl LIRC_SET_MEASURE_CARRIER_MODE
+***********************************
+
+Name
+====
+
+LIRC_SET_MEASURE_CARRIER_MODE - enable or disable measure mode
+
+Synopsis
+========
+
+.. c:function:: int ioctl( int fd, LIRC_SET_MEASURE_CARRIER_MODE, __u32 *enable )
+    :name: LIRC_SET_MEASURE_CARRIER_MODE
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by open().
+
+``enable``
+    enable = 1 means enable measure mode, enable = 0 means disable measure
+    mode.
+
+
+Description
+===========
+
+.. _lirc-mode2-frequency:
+
+Enable or disable measure mode. If enabled, from the next key
+press on, the driver will send ``LIRC_MODE2_FREQUENCY`` packets. By
+default this should be turned off.
+
+
+Return Value
+============
+
+On success 0 is returned, on error -1 and the ``errno`` variable is set
+appropriately. The generic error codes are described at the
+:ref:`Generic Error Codes <gen-errors>` chapter.