Documentation claims toxav_iteration_interval is threadsafe but it's not

[sudden6]

https://github.com/TokTok/c-toxcore/blob/master/toxav/toxav.h#L52 Claims the API is fully threadsafe, but https://github.com/TokTok/c-toxcore/blob/master/toxav/toxav.c#L237 accesses ToxAV data structures without proper locking.

I think I accidentally fixed this by changing the docs in #1307 but I think we should also use proper locking here.

[sudden6]

fixed the ToxAV documentation in 683bc80