HashSet Docs: Split First Paragraph

This way, the module index renders only the first sentence as a short
description.

src/libstd/collections/hash/set.rs

@@ -31,10 +31,12 @@
 // to get rid of it properly.
 
 /// An implementation of a hash set using the underlying representation of a
-/// HashMap where the value is (). As with the `HashMap` type, a `HashSet`
-/// requires that the elements implement the `Eq` and `Hash` traits. This can
-/// frequently be achieved by using `#[derive(Eq, Hash)]`. If you implement
-/// these yourself, it is important that the following property holds:
+/// HashMap where the value is ().
+///
+/// As with the `HashMap` type, a `HashSet` requires that the elements
+/// implement the `Eq` and `Hash` traits. This can frequently be achieved by
+/// using `#[derive(Eq, Hash)]`. If you implement these yourself, it is
+/// important that the following property holds:
 ///
 /// ```text
 /// k1 == k2 -> hash(k1) == hash(k2)