media: dev-encoder.rst: add reference to V4L2_FMT_FLAG_ENC_CAP_FRAME_INTERVAL

Setting the stateful encoder capture frame interval is only supported
if this flag is set. Document this.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/dev-encoder.rst

@@ -281,7 +281,8 @@ Initialization
 5. **Optional** Set the coded frame interval on the ``CAPTURE`` queue via
    :c:func:`VIDIOC_S_PARM`. This is only necessary if the coded frame
    interval is different from the raw frame interval, which is typically
-   the case for off-line encoding.
+   the case for off-line encoding. Support for this feature is signalled
+   by the :ref:`V4L2_FMT_FLAG_ENC_CAP_FRAME_INTERVAL <fmtdesc-flags>` format flag.
 
    * ** Required fields:**