Update documentation for websocket::permessage_deflate

Resolves #962

include/boost/beast/websocket/option.hpp

@@ -21,8 +21,13 @@ namespace websocket {
     These settings control the permessage-deflate extension,
     which allows messages to be compressed.
 
-    @note Objects of this type are used with
-          @ref beast::websocket::stream::set_option.
+    @note
+
+    These settings should be configured before performing the WebSocket
+    handshake.
+
+    Objects of this type are used with
+    @ref beast::websocket::stream::set_option.
 */
 struct permessage_deflate
 {

include/boost/beast/websocket/stream.hpp

@@ -398,6 +398,11 @@ public:
 
         @throws invalid_argument if `deflateSupported == false`, and either
         `client_enable` or `server_enable` is `true`.
+
+        @note
+
+        These settings should be configured before performing the WebSocket
+        handshake.
     */
     void
     set_option(permessage_deflate const& o);