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>