media: docs: dev-stateless-decoder: Document frame type flags

Document that frame type flags must be set for the coded buffer.
Decoders, like NVIDIA Tegra h264 decoder for example, won't work
properly without these flags.

Signed-off-by: Dmitry Osipenko <[email protected]>

Author: digetx

Documentation/userspace-api/media/v4l/dev-stateless-decoder.rst

@@ -271,6 +271,9 @@ A typical frame would thus be decoded using the following sequence:
           we are not sure that the current decode request is the last one needed
           to produce a fully decoded frame, then
           ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` must also be set.
+          ``V4L2_BUF_FLAG_KEYFRAME``, ``V4L2_BUF_FLAG_PFRAME`` and
+          ``V4L2_BUF_FLAG_BFRAME`` must be set if relevant to the coded
+          format.
 
       ``request_fd``
           must be set to the file descriptor of the decoding request.