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/backtrace.rs

@@ -271,7 +271,7 @@ impl Backtrace {
         enabled
     }
 
-    /// Capture a stack backtrace of the current thread.
+    /// Captures a stack backtrace of the current thread.
     ///
     /// This function will capture a stack backtrace of the current OS thread of
     /// execution, returning a `Backtrace` type which can be later used to print