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-parser/src/parser.rs

@@ -5926,7 +5926,7 @@ pub fn discover_captures_in_closure(
             seen.push(var_id);
         }
     }
-    for positional in &block.signature.rest_positional {
+    if let Some(positional) = &block.signature.rest_positional {
         if let Some(var_id) = positional.var_id {
             seen.push(var_id);
         }