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/async_iter/async_iter.rs

@@ -18,7 +18,7 @@ pub trait AsyncIterator {
     /// The type of items yielded by the async iterator.
     type Item;
 
-    /// Attempt to pull out the next value of this async iterator, registering the
+    /// Attempts to pull out the next value of this async iterator, registering the
     /// current task for wakeup if the value is not yet available, and returning
     /// `None` if the async iterator is exhausted.
     ///
@@ -139,7 +139,7 @@ impl<T> Poll<Option<T>> {
     pub const FINISHED: Self = Poll::Ready(None);
 }
 
-/// Convert something into an async iterator
+/// Converts something into an async iterator
 #[unstable(feature = "async_iterator", issue = "79024")]
 pub trait IntoAsyncIterator {
     /// The type of the item yielded by the iterator