create user alloc for http if requested

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/libwebsockets-api-doc.html b/libwebsockets-api-doc.html
index 17d9e5b..b041e74 100644
--- a/libwebsockets-api-doc.html
+++ b/libwebsockets-api-doc.html
@@ -611,6 +611,11 @@
 total number of client connections allowed set
 by MAX_CLIENTS.
 </blockquote>
+<h3>LWS_CALLBACK_HTTP_WRITEABLE</h3>
+<blockquote>
+you can write more down the http protocol
+link now.
+</blockquote>
 <h3>LWS_CALLBACK_HTTP_FILE_COMPLETION</h3>
 <blockquote>
 a file requested to be send down