Document lock usage in GstVideoDecoder
As follow up of gst-plugins-base#358 (closed), it may be useful to add this to the documentation of GstVideoDecoder, in relation to GST_VIDEO_DECODER_STREAM_LOCK.
The decoder stream lock is likely redundant, but we are stuck with it, since it's in the API/Abi.
That lock though should always be dropped when a flush or a transition to ready State occurs. Subclass are > responsible for that if these subclass have a separated thread, like omxvideodec and v4l2videodec.
Please, feel free to close this issue, if it is not worth it.