doc/encoders: improve libvo-aacenc doc

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
(cherry picked from commit 81bbe49a0e588aa899f37a567808ba8926d798d6)

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