Clippy fixes from stable and nightly (#13455)

- **Doccomment style fixes**
- **Forgotten stuff in `nu-pretty-hex`**
- **Don't `for` around an `Option`**
- and more

I think the suggestions here are a net positive, some of the suggestions
moved into #13498 feel somewhat arbitrary, I also raised
https://github.com/rust-lang/rust-clippy/issues/13188 as the nightly
`byte_char_slices` would require either a global allow or otherwise a
ton of granular allows or possibly confusing bytestring literals.

crates/nu-system/src/foreground.rs

@@ -61,9 +61,8 @@ impl ForegroundChild {
                         pipeline_state: Some(pipeline_state.clone()),
                     }
                 })
-                .map_err(|e| {
+                .inspect_err(|_e| {
                     foreground_pgroup::reset();
-                    e
                 })
         } else {
             command.spawn().map(|child| Self {