Rollup merge of #105744 - Ezrashaw:e0158-clarity, r=GuillaumeGomez

Rewrite `E0158` error-code docs for clarity

Fixes #105585.

The `E0158` error-code docs are unclear. It doesn't explain all three different variants of the error and doesn't explain *why* the error occurs. This PR cleans it up a bit and brings it properly into line with [RFC1567](https://rust-lang.github.io/rfcs/1567-long-error-codes-explanation-normalization.html).

I'm a first time Rust contributor so I've probably not got it quite right. I also haven't run the whole build process because I assume that my minor docs changes shouldn't break everything.