Update the documentation for theora_encode_comment().

It wrongly claimed the returned buffer was owned by libtheora, when
 in reality libtheora was creating a temporary buffer and not
 storing the pointer to the returned packet anywhere.
Since there's no encoder parameter passed to this function, it
 doesn't even have a place to store it.
This situation is sub-optimal, but can be solved by migrating to
 the 1.0 API, which does require an encoder instance to encode a
 comment, and thus does not have this problem.

Fixes #726.

svn path=/trunk/theora/; revision=17994