aacenc: fix option descriptions

Since the new PNS implementation has been merged and is no longer considered
proof of concept (as it's much more complex and better than the previous), change
the comments to reflect that. We need people testing it (since all AAC profiles
require it to be on by default) and having it tagged as proof of concept might drive some away.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
