lavfi/vidstabtransform: apply various documentation/option minor fixes

Improve consistency with the rest of the documentation.
doc/muxers/segment: fix formula for computing the segment_time_delta value
doc/encoders: add libx264rgb doc and supported pixfmts for libx264(rgb)

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/muxers: add "Options", "Examples", "Syntax", etc. subsections

More consistent with codec docs. Also some minor reorganization of sentences
to fit the new document hierarchy.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
avcodec/mjpegdec: Dont skip picture allocation if theres no picture allocated

Fixes Ticket 3245