Fix some nightly lints (#15028)
Apply fixes for some `cargo check` and `cargo clippy` lints that are on
in nightly Rust.
The following command now passes, the blanket allows had to many
false-positives:
```
cargo +nightly clippy -- -A clippy::doc_markdown -A mismatched_lifetime_syntaxes -A clippy::explicit_deref_methods
```
`cargo +nightly check -- -A mismatched_lifetime_syntaxes` now passes
without warnings.
