Add module documentation for rustdoc passes

These are currently documented at
https://rustc-dev-guide.rust-lang.org/rustdoc-internals.html#hot-potato
but can easily go out of date. We'd like to document them in place and
link to
https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/passes/index.html

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -1,3 +1,4 @@
+//! Calculates information used for the --show-coverage flag.
 use crate::clean;
 use crate::core::DocContext;
 use crate::html::markdown::{find_testable_code, ErrorCodes};