docs/egl: add some more documentation

Inspired by `src/egl/main/README.txt`, which was severely outdated, but
still contained valid information.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6130>
2 files changed