resolve: Add some comments to the main modules

src/librustc_resolve/build_reduced_graph.rs

-//! Reduced graph building.
+//! After we obtain a fresh AST fragment from a macro, code in this module helps to integrate
+//! that fragment into the module structures that are already partially built.
 //!
-//! Here we build the "reduced graph": the graph of the module tree without
-//! any imports resolved.
+//! Items from the fragment are placed into modules,
+//! unexpanded macros in the fragment are visited and registered.
+//! Imports are also considered items and placed into modules here, but not resolved yet.
 
 use crate::macros::{LegacyBinding, LegacyScope};
 use crate::resolve_imports::ImportDirective;

src/librustc_resolve/late.rs

+//! "Late resolution" is the pass that resolves most of names in a crate beside imports and macros.
+//! It runs when the crate is fully expanded and its module structure is fully built.
+//! So it just walks through the crate and resolves all the expressions, types, etc.
+//!
+//! If you wonder why there's no `early.rs`, that's because it's split into three files -
+//! `build_reduced_graph.rs`, `macros.rs` and `resolve_imports.rs`.
+
 use GenericParameters::*;
 use RibKind::*;
 

src/librustc_resolve/lib.rs

+//! This crate is responsible for the part of name resolution that doesn't require type checker.
+//!
+//! Module structure of the crate is built here.
+//! Paths in macros, imports, expressions, types, patterns are resolved here.
+//! Label names are resolved here as well.
+//!
+//! Type-relative name resolution (methods, fields, associated items) happens in `librustc_typeck`.
+//! Lifetime names are resolved in `librustc/middle/resolve_lifetime.rs`.
+
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
 
 #![feature(crate_visibility_modifier)]

src/librustc_resolve/macros.rs

+//! A bunch of methods and structures more or less related to resolving macros and
+//! interface provided by `Resolver` to macro expander.
+
 use crate::{AmbiguityError, AmbiguityKind, AmbiguityErrorMisc, Determinacy};
 use crate::{CrateLint, Resolver, ResolutionError, Scope, ScopeSet, ParentScope, Weak};
 use crate::{ModuleKind, NameBinding, PathResult, Segment, ToNameBinding};

src/librustc_resolve/resolve_imports.rs

+//! A bunch of methods and structures more or less related to resolving imports.
+
 use ImportDirectiveSubclass::*;
 
 use crate::{AmbiguityError, AmbiguityKind, AmbiguityErrorMisc};