Document how a RequestBuilder gets constructed (#1097)

src/blocking/request.rs

@@ -22,6 +22,8 @@ pub struct Request {
 }
 
 /// A builder to construct the properties of a `Request`.
+///
+/// To construct a `RequestBuilder`, refer to the `Client` documentation.
 #[derive(Debug)]
 #[must_use = "RequestBuilder does nothing until you 'send' it"]
 pub struct RequestBuilder {