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.

library/std/tests/create_dir_all_bare.rs

@@ -2,6 +2,7 @@
 
 //! Note that this test changes the current directory so
 //! should not be in the same process as other tests.
+
 use std::env;
 use std::fs;
 use std::path::{Path, PathBuf};