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/core/src/macros/mod.rs

@@ -633,7 +633,7 @@ macro_rules! write {
     };
 }
 
-/// Write formatted data into a buffer, with a newline appended.
+/// Writes formatted data into a buffer, with a newline appended.
 ///
 /// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
 /// (no additional CARRIAGE RETURN (`\r`/`U+000D`).