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.

Author: nnethercote

crates/core_simd/examples/dot_product.rs

@@ -1,6 +1,5 @@
-// Code taken from the `packed_simd` crate
-// Run this code with `cargo test --example dot_product`
-//use std::iter::zip;
+//! Code taken from the `packed_simd` crate.
+//! Run this code with `cargo test --example dot_product`.
 
 #![feature(array_chunks)]
 #![feature(slice_as_chunks)]