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/panic.rs

@@ -160,7 +160,7 @@ pub unsafe trait PanicPayload: crate::fmt::Display {
     /// Just borrow the contents.
     fn get(&mut self) -> &(dyn Any + Send);
 
-    /// Try to borrow the contents as `&str`, if possible without doing any allocations.
+    /// Tries to borrow the contents as `&str`, if possible without doing any allocations.
     fn as_str(&mut self) -> Option<&str> {
         None
     }