DOC: lua: Add a warning about buffers modification in HTTP

Since the 1.9, it is forbidden to alter the channel buffer from an HTTP
stream because there is no way to keep the HTTP parser synchronized if the
buffer content is altered. In addition, since the HTX is the only
reprensentation for HTTP messages, the data in HTTP buffers are structured
and cannot be read or updated in a raw fashion.

A warning is triggered when a user tries to alter an HTTP buffer. However,
it was not documented. This patch adds a warning in the lua documentation.

This patch is related to the issue #1287. It may be backported as far as
2.0.

doc/lua-api/index.rst

@@ -1281,6 +1281,10 @@ Channel class
   **Warning**: It is not possible to read from the response in request action,
   and it is not possible to read for the request channel in response action.
 
+  **Warning**: It is forbidden to alter the Channels buffer from HTTP contexts.
+  So only :js:func:`Channel.get_in_length`, :js:func:`Channel.get_out_length`
+  and :js:func:`Channel.is_full` can be called from an HTTP conetext.
+
 .. image:: _static/channel.png
 
 .. js:function:: Channel.dup(channel)