Convert some module-level // and /// comments to //!.

This makes their intent and expected location clearer. We see some
examples where these comments were not clearly separate from `use`
declarations, which made it hard to understand what the comment is
describing.

compiler/rustc_middle/src/mir/statement.rs

@@ -1,4 +1,5 @@
-/// Functionality for statements, operands, places, and things that appear in them.
+//! Functionality for statements, operands, places, and things that appear in them.
+
 use super::{interpret::GlobalAlloc, *};
 
 ///////////////////////////////////////////////////////////////////////////