Closes#3266.
This updates our typing-features and typing-faq docs to account for
recent changes in ty's behavior. Both the "gradual guarantee" and
"fixpoint iteration" sections no longer describe our actual behavior. In
both cases, I tried different ways to update the section, but ended up
feeling like there was nothing that distinctive or interesting to say,
so I just removed the sections entirely.
For gradual guarantee, I don't think we should have a section with that
name, since in general we don't take that as a guiding principle
anymore. The remaining singleton case is very niche, and already
discussed in the FAQ.
For fixpoint iteration, we still do that of course, but for performance
reasons we have since decided to promote literal unions much more
quickly in all cyclic cases. So I wasn't able to construct any realistic
case anymore where this "union of literals up to some limit" behavior is
visible.
## Summary
Now that we do not union in `Unknown` into element types of invariant
collections anymore, this question comes up more frequently. I'm taking
a first stab at trying to explain this in a concise way. I am also
planning to add a diagnostic hint that would be emitted in those
situations, which could refer to this FAQ entry.
Tobias Hernstig