Rollup merge of #136687 - joshtriplett:improve-display-and-fromstr-docs, r=Amanieu

Improve the documentation of `Display` and `FromStr`, and their interactions

In particular:
- `Display` is not necessarily lossless
- The output of `Display` might not be parseable by `FromStr`, and might
  not produce the same value if it is.
- Calling `.parse()` on the output of `Display` is usually a mistake
  unless a type's documented output and input formats match.
- The input formats accepted by `FromStr` depend on the type.

This documentation adds no API surface area and makes no guarantees about stability. To the best of my knowledge, everything it says is already established to be true. As such, I don't think it needs an FCP.

Author: matthiaskrgr