anidb: Document which backend API is used on the Get* methods

Queries that depend on the UDP API can't be used without SetCredentials.
anidb: Document that credentials are needed for UDP queries

Commit messages aren't documentation :(
anidb: Always remove credentials on Logout()

Even if they were never used.
anidb: Add missing loop 'break's to EpisodeByEID

Without the breaks, it's would be possible to query for an Anime
with AID 0, a clear waste of time.

Also move the 'ok = false' assignment closer to where it's documented.