The docs do appear to be somewhat incomplete, though, there's functionality used in the examples that the docs otherwise don't seem to mention even exists.

I've used Moviepy fairly extensively in a recent project and yes, the docs are incomplete. I've had to resort to actually reading the source code to figure out why, for example, my argument was being ignored. I still have some unresolved (though seemingly obscure) issues with transparency, and my question to the moviepy subreddit went unanswered.