Document all public items in rustc_incremental

Also:

- Review and edit current docs
- Enforce documentation for crate

Co-authored-by: r00ster <r00ster91@protonmail.com>
Co-authored-by: Camille Gillot <gillot.camille@gmail.com>

compiler/rustc_incremental/src/assert_module_sources.rs

@@ -29,6 +29,7 @@ use rustc_session::cgu_reuse_tracker::*;
 use rustc_span::symbol::{sym, Symbol};
 use std::collections::BTreeSet;
 
+#[allow(missing_docs)]
 pub fn assert_module_sources(tcx: TyCtxt<'_>) {
     tcx.dep_graph.with_ignore(|| {
         if tcx.sess.opts.incremental.is_none() {