Add leading sentence for <unordered_set> operators reference

Author: Rageking8

docs/standard-library/unordered-set-operators.md

@@ -7,6 +7,8 @@ ms.assetid: 8653eea6-12f2-4dd7-aa2f-db38a71599a0
 ---
 # `<unordered_set>` operators
 
+The `<unordered_set>` header provides the following operators:
+
 ## <a name="op_neq"></a> `operator!=`
 
 Tests whether the [`unordered_set`](../standard-library/unordered-set-class.md) object on the left side of the operator is not equal to the `unordered_set` object on the right side.