Be more detailed in documentation of set_query (#737)

* Be more detailed in documentation of set_query

* Remove trailing space

---------

Co-authored-by: Martin Robinson <mrobinson@igalia.com>

url/src/lib.rs

@@ -1524,7 +1524,8 @@ impl Url {
         }
     }
 
-    /// Change this URL’s query string.
+    /// Change this URL’s query string. If `query` is `None`, this URL's
+    /// query string will be cleared.
     ///
     /// # Examples
     ///