Add blank lines after module-level //! comments.

Most modules have such a blank line, but some don't. Inserting the blank
line makes it clearer that the `//!` comments are describing the entire
module, rather than the `use` declaration(s) that immediately follows.

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -1,4 +1,5 @@
 //! Calculates information used for the --show-coverage flag.
+
 use crate::clean;
 use crate::core::DocContext;
 use crate::html::markdown::{find_testable_code, ErrorCodes};