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/engine/mod.rs

@@ -1,3 +1,4 @@
+//! Representation of the engine state and many of the details that implement the scoping
 mod argument;
 mod cached_file;
 mod call;