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-command/src/conversions/fill.rs

@@ -150,13 +150,9 @@ fn fill(
         FillAlignment::Left
     };
 
-    let width = if let Some(arg) = width_arg { arg } else { 1 };
+    let width = width_arg.unwrap_or(1);
 
-    let character = if let Some(arg) = character_arg {
-        arg
-    } else {
-        " ".to_string()
-    };
+    let character = character_arg.unwrap_or_else(|| " ".to_string());
 
     let arg = Arguments {
         width,