media: docs-rst: Document EBUSY for VIDIOC_S_FMT

VIDIOC_S_FMT may return EBUSY if the device is streaming or there are
buffers allocated. Document this.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/vidioc-g-fmt.rst

@@ -147,3 +147,9 @@ appropriately. The generic error codes are described at the
 EINVAL
     The struct :c:type:`v4l2_format` ``type`` field is
     invalid or the requested buffer type not supported.
+
+EBUSY
+    The device is busy and cannot change the format. This could be
+    because or the device is streaming or buffers are allocated or
+    queued to the driver. Relevant for :ref:`VIDIOC_S_FMT
+    <VIDIOC_G_FMT>` only.