doc/encoders: reformat libmp3lame doc

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
(cherry picked from commit 40b8350b57adaa9aaf6731bac5130d4fec1639c3)

Conflicts:
	doc/encoders.texi
doc/encoders: Remove options that were not there when branch was cut from master

Signed-off-by: Timothy Gu <timothygu99@gmail.com>