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-send-duty-cycle.rst

@@ -0,0 +1,54 @@
+.. 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_send_duty_cycle:
+
+******************************
+ioctl LIRC_SET_SEND_DUTY_CYCLE
+******************************
+
+Name
+====
+
+LIRC_SET_SEND_DUTY_CYCLE - Set the duty cycle of the carrier signal for
+IR transmit.
+
+Synopsis
+========
+
+.. c:function:: int ioctl( int fd, LIRC_SET_SEND_DUTY_CYCLE, __u32 *duty_cycle)
+    :name: LIRC_SET_SEND_DUTY_CYCLE
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by open().
+
+``duty_cycle``
+    Duty cicle, describing the pulse width in percent (from 1 to 99) of
+    the total cycle. Values 0 and 100 are reserved.
+
+
+Description
+===========
+
+Get/set the duty cycle of the carrier signal for IR transmit.
+
+Currently, no special meaning is defined for 0 or 100, but this
+could be used to switch off carrier generation in the future, so
+these values should be reserved.
+
+
+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.