[media] doc-rst: document ioctl LIRC_GET_SEND_MODE

Move the documentation of this ioctl from lirc_ioctl to its
own file, and add a short description about the pulse mode
used by IR TX.

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

Documentation/media/uapi/rc/lirc-get-send-mode.rst

@@ -0,0 +1,48 @@
+.. -*- coding: utf-8; mode: rst -*-
+
+.. _lirc_get_send_mode:
+
+************************
+ioctl LIRC_GET_SEND_MODE
+************************
+
+Name
+====
+
+LIRC_GET_SEND_MODE - Get supported transmit mode.
+
+Synopsis
+========
+
+.. cpp:function:: int ioctl( int fd, int request, __u32 *tx_modes )
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by open().
+
+``request``
+    LIRC_GET_SEND_MODE
+
+``tx_modes``
+    Bitmask with the supported transmit modes.
+
+
+Description
+===========
+
+Get supported transmit mode.
+
+.. _lirc-mode-pulse:
+
+Currently, only ``LIRC_MODE_PULSE`` is supported by lircd on TX. On
+puse mode, a sequence of pulse/space integer values are written to the
+lirc device using ``write()``.
+
+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.