udp: try to clarify verbiage on buffer_size

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/snowenc: make snow non experimental

After this commit the created bitstream will be supported by future
decoders.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>