Convert some notes to help messages

Closes #18126.

At the moment this mostly only changes notes that are particularly help-oriented or directly suggest the user to do something to help messages, and does not change messages that simply explain an error message further. If it is decided that those messages should also be help messages, I can add them to this PR, but for now I’m excluding them as I believe that changing those messages might leave very few places where notes would be appropriate.
Yes, it generically supports [any kind of message](https://github.com/rust-lang/rust/blob/master/src/compiletest/errors.rs#L20); I could add a new type of message, `foo`, to the compiler, and it would automatically support them. There are plenty of tests which use `NOTE`, and yet there are no mentions of the word `note` in `src/compiletest` other than a few debug print messages.

(Plus, I tested it with an invalid message.)