Document how/when to use diagnostic items

[xFrednet]

Rustc has introduced diagnostic items to avoid using hard-coded paths to check for std and core types. However, there seems to be a lack of documentation regarding them. Here is a rough overview of what should be documented:

• How to find the diagnostic item for type XYZ
• How to add a new diagnostic item
  • Define diagnostic item naming conventions.
• How to use diagnostic items

See rust-lang/rust-clippy#5393 for more context and related discussions/links etc.

[xFrednet]

I'll start working on this. But it might take some time as I'm also busy with some RL stuff currently 🙃

[camelid]

Assigned you so that people know you're planning to work on it in the future :)

[xFrednet]

Thank you!

[LeSeulArtichaut]

(@rustbot claim is enabled in this repo as well, in case you would want to work on something else in the future 🙃)