Rollup merge of #68096 - varkor:diagnostic-cleanup, r=Centril

Rollup merge of #68096 - varkor:diagnostic-cleanup, r=Centril

Clean up some diagnostics by making them more consistent

In general:

- Diagnostic should start with a lowercase letter.
- Diagnostics should not end with a full stop.
- Ellipses contain three dots.
- Backticks should encode Rust code.

I also reworded a couple of messages to make them read more clearly.

It might be sensible to create a style guide for diagnostics, so these informal conventions are written down somewhere, after which we could audit the existing diagnostics.

r? @Centril