Improve the usage message for `-Z dump-mir`.

Ouput now looks like this:
```
$ rustc -Z help
...
    -Z               arg-align-attributes -- emit align metadata for reference arguments
    -Z                       dump-mir=val -- dump MIR state to file.
        `val` is used to select which passes and functions to dump. For example:
        `all` matches all passes and functions,
        `foo` matches all passes for functions whose name contains 'foo',
        `foo & ConstProp` only the 'ConstProp' pass for function names containing 'foo',
        `foo | bar` all passes for function names containing 'foo' or 'bar'.
    -Z                   dump-mir-dir=val -- the directory the MIR is dumped into
...
```

Fixes rust-lang#56558

Rollup of 6 pull requests

Successful merges:

 - #56248 (Suggest an appropriate token when encountering `pub Ident<'a>`)
 - #56597 (Improve the usage message for `-Z dump-mir`.)
 - #56599 (codegen: Fix va_list - aarch64 iOS/Windows)
 - #56602 (Fix the just-introduced ptr::hash docs)
 - #56620 (resolve: Reduce some clutter in import ambiguity errors)
 - #56621 (Add missing comma in Generators)

Failed merges:

r? @ghost