aacenc: use constants to set AAC coder

The names of the constants comes from the ff_aac_coders array in aaccoder.c.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/encoders: add doc for AAC encoder

Thanks-to: Kostya Shishkov <kostya.shishkov@gmail.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/encoders: Improve libvo-aacenc doc

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