Add a pinch of comments

crates/ra_syntax/src/ast/generated.rs

@@ -1,6 +1,12 @@
 // This file is automatically generated based on the file `./generated.rs.tera` when `cargo gen-syntax` is run
 // Do not edit manually
 
+//! This module contains auto-generated Rust AST. Like `SyntaxNode`s, AST nodes
+//! are generic over ownership: `X<'a>` things are `Copy` references, `XNode`
+//! are Arc-based. You can switch between the two variants using `.owned` and
+//! `.borrowed` functions. Most of the code works with borowed mode, and only
+//! this mode has all AST accessors.
+
 #![cfg_attr(rustfmt, rustfmt_skip)]
 
 use crate::{