Add docs

Author: oxalica

crates/ra_cfg/src/cfg_expr.rs

@@ -1,3 +1,7 @@
+//! The condition expression used in `#[cfg(..)]` attributes.
+//!
+//! See: https://doc.rust-lang.org/reference/conditional-compilation.html#conditional-compilation
+
 use std::slice::Iter as SliceIter;
 
 use ra_syntax::SmolStr;

crates/ra_hir/src/attr.rs

@@ -1,3 +1,5 @@
+//! A higher level attributes based on TokenTree, with also some shortcuts.
+
 use std::sync::Arc;
 
 use mbe::ast_to_token_tree;