[media] doc-rst: document LIRC_SET_TRANSMITTER_MASK

Add proper documentation for this ioctl, providing some
additional information about its usage.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/rc/lirc-set-transmitter-mask.rst

@@ -0,0 +1,53 @@
+.. -*- coding: utf-8; mode: rst -*-
+
+.. _lirc_set_transmitter_mask:
+
+*******************************
+ioctl LIRC_SET_TRANSMITTER_MASK
+*******************************
+
+Name
+====
+
+LIRC_SET_TRANSMITTER_MASK - Enables send codes on a given set of transmitters
+
+Synopsis
+========
+
+.. cpp:function:: int ioctl( int fd, int request, __u32 *mask )
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by open().
+
+``request``
+    LIRC_SET_TRANSMITTER_MASK
+
+``mask``
+    Mask with channels to enable tx. Channel 0 is the least significant bit.
+
+
+Description
+===========
+
+Some IR TX devices have multiple output channels, in such case,
+:ref:`LIRC_CAN_SET_TRANSMITTER_MASK <LIRC-CAN-SET-TRANSMITTER-MASK>` is
+returned via :ref:`LIRC_GET_FEATURES` and this ioctl sets what channels will
+send IR codes.
+
+This ioctl enables the given set of transmitters. The first transmitter is
+encoded by the least significant bit and so on.
+
+When an invalid bit mask is given, i.e. a bit is set, even though the device
+does not have so many transitters, then this ioctl returns the number of
+available transitters and does nothing otherwise.
+
+
+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.