docs(client): better describe Uri forms are sent as-is

Closes #3693

src/client/conn/http1.rs

@@ -176,8 +176,16 @@ where
     ///
     /// `req` must have a `Host` header.
     ///
-    /// Absolute-form `Uri`s are not required. If received, they will be serialized
-    /// as-is.
+    /// # Uri
+    ///
+    /// The `Uri` of the request is serialized as-is.
+    ///
+    /// - Usually you want origin-form (`/path?query`).
+    /// - For sending to an HTTP proxy, you want to send in absolute-form
+    ///   (`https://hyper.rs/guides`).
+    ///
+    /// This is however not enforced or validated and it is up to the user
+    /// of this method to ensure the `Uri` is correct for their intended purpose.
     pub fn send_request(
         &mut self,
         req: Request<B>,