Document public types in nu-protocol (#12906)

- **Doc-comment public `nu-protocol` modules**
- **Doccomment argument/signature/call stuff**
- **Doccomment cell path types**
- **Doccomment expression stuff**
- **Doccomment import patterns**
- **Doccomment pattern matching AST nodes**

crates/nu-protocol/src/ast/expression.rs

@@ -6,6 +6,7 @@ use crate::{
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 
+/// Wrapper around [`Expr`]
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Expression {
     pub expr: Expr,