doc/codecs: extend documentation for thread_type option

Based on avcodec.h documentation.
lavc/h264_slice: clarify informational message in case of disabled slice multi-threaded decoding

Also suggest what can be done to workaround the limitation.
doc: remove RELEASE_NOTES

It hasn't been used since 0.11, and it does not contain any useful
information except for the code name.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>