add minimal comments

crates/ra_syntax/src/ast/expr_extensions.rs

@@ -1,3 +1,5 @@
+//! Various extension methods to ast Expr Nodes, which are hard to code-generate.
+
 use crate::{
     SyntaxToken, SyntaxElement, SmolStr,
     ast::{self, AstNode, AstChildren, children, child_opt},

crates/ra_syntax/src/ast/extensions.rs

@@ -1,3 +1,6 @@
+//! Various extension methods to ast Nodes, which are hard to code-generate.
+//! Extensions for various expressions live in a sibling `expr_extensions` module.
+
 use itertools::Itertools;
 
 use crate::{

crates/ra_syntax/src/ast/tokens.rs

@@ -1,3 +1,5 @@
+//! There are many AstNodes, but only a few tokens, so we hand-write them here.
+
 use crate::{
     SyntaxToken,
     SyntaxKind::{COMMENT, WHITESPACE},

crates/ra_syntax/src/ast/traits.rs

@@ -1,3 +1,7 @@
+//! Various traits that are implemented by ast nodes.
+//!
+//! The implementations are usually trivial, and live in generated.rs
+
 use itertools::Itertools;
 
 use crate::{