Hallo!
It's nice to have both. If you have a well documented API it's much easier to decipher the source, since you can clearly see a description of a function, its arguments, its return value, and what file it resides in.
of course ... you are right.
bit no one will bother you. it you start an api documentation then you should find a form that allows others to maintain it with you (wiki?)
maybe this is the way to go ? We could use the PD-Wiki at http://pd.iem.at/pdwiki/ ?
LG Georg