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/io/error.rs

@@ -167,7 +167,7 @@ impl SimpleMessage {
     }
 }
 
-/// Create and return an `io::Error` for a given `ErrorKind` and constant
+/// Creates and returns an `io::Error` for a given `ErrorKind` and constant
 /// message. This doesn't allocate.
 pub(crate) macro const_io_error($kind:expr, $message:expr $(,)?) {
     $crate::io::error::Error::from_static_message({
@@ -852,7 +852,7 @@ impl Error {
         }
     }
 
-    /// Attempt to downcast the custom boxed error to `E`.
+    /// Attempts to downcast the custom boxed error to `E`.
     ///
     /// If this [`Error`] contains a custom boxed error,
     /// then it would attempt downcasting on the boxed error,