mp3enc: Properly write bitrate value in XING header

Instead of using a fixed bitrate_idx, calculate a matching bitrate for
the XING header.

Using a fixed bitrate_idx causes tools such as file(1) and mediainfo(1)
to report wrong bitrate and bitrate mode when using CBR.

Bug-Id: https://bugs.debian.org/736088

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
doc: Libavfilter English cleanup

This aims to standardize the English used in the libavfilter
documentation, describing the same thing in the same way more often,
and significantly reduce the amount of outright errors, including
run-on sentences and sentence fragments.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>