DOC: lua-api: Remove warning about the lua filters

The lua filters api is no longer experimental. There are some filters
depending on this api, thus it is fair to make it stable now.
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index 70dbf63..2f2744f 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -72,13 +72,6 @@
    `core.register_filter()`. Each declared filter is prefixed by the string
    "lua.".
 
-   .. warning::
-      The Lua filter support is highly experimental. The API is still unstable
-      and may change without notice. No backward compatibility should be
-      expected for now. Use it with an extreme caution and report any issue or
-      comment about it. The feature was unveiled to improve it and to adapt it
-      to real usages.
-
 
 HAProxy Lua Hello world
 -----------------------