avcodec: doxy: Clarify what get_buffer2 must fill in buf[]

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
avcodec: doxy: Clarify how to use buf[] when implementing get_buffer2()

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
avutil: doxy: Small clarification for av_buffer_create()

Explicitly saying it can't hurt.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>