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/error.rs

@@ -234,7 +234,7 @@ impl<E> Report<E>
 where
     Report<E>: From<E>,
 {
-    /// Create a new `Report` from an input error.
+    /// Creates a new `Report` from an input error.
     #[unstable(feature = "error_reporter", issue = "90172")]
     pub fn new(error: E) -> Report<E> {
         Self::from(error)