Fix doc nits

Many tiny changes to stdlib doc comments to make them consistent (for example
"Returns foo", rather than "Return foo", per RFC1574), adding missing periods, paragraph
breaks, backticks for monospace style, and other minor nits.

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text

library/std/src/sys_common/wstr.rs

@@ -15,7 +15,7 @@ pub struct WStrUnits<'a> {
 }
 
 impl WStrUnits<'_> {
-    /// Create the iterator. Returns `None` if `lpwstr` is null.
+    /// Creates the iterator. Returns `None` if `lpwstr` is null.
     ///
     /// SAFETY: `lpwstr` must point to a null-terminated wide string that lives
     /// at least as long as the lifetime of this struct.