Fix unbound references in API documentation

[julienrf]

• IterableOnceOps (introduced in Introduce IterableOnceOps as common abstraction for Iterable & Iterator #480) produces a lot of documentation errors. We should check that the documentation variables are always defined before they are used,
• Also, now Iterable inherits from a lot of comments that are relevant to IterableOnce only (about the fact that an operation consumes the collection or not). We should override these comments (or redefine the variables they use).

[szeiger]

Unassigning myself for now because I haven't gotten around to starting this.

[chrisphelps]

I'd like to take a look at this one. I should get to it in the next couple of days.

[julienrf]

Woohoo that would be awesome! You can do two separate PRs if that’s easier :)