DOC: httpclient: document status 0 on internal error

This patch documents the behavior where the internal HTTP client sets
the response status to 0 when an error is encountered by the stream
(SF_ERR_MASK).

This allows users to distinguish between an HTTP status code returned
by a remote server and an internal error generated by HAProxy (e.g.
connection timeout, connection refused, etc.).

doc/lua-api/index.rst

@@ -2614,7 +2614,8 @@ HTTPClient class
    haproxy address format.
   :param integer request.timeout: Optional timeout parameter, set a
    "timeout server" on the connections.
-  :returns: Lua table containing the response
+  :returns: Lua table containing the response. If an internal error occurs (e.g.
+   connection failure, timeout, etc.), the ``status`` field will be set to 0.
 
 
 .. code-block:: lua

doc/management.txt

@@ -2469,7 +2469,8 @@ httpclient [--htx] <method> <URI>
   name in the URL using the "default" resolvers section, which is populated
   with the DNS servers of your /etc/resolv.conf by default. However it won't be
   able to resolve an host from /etc/hosts if you don't use a local dns daemon
-  which can resolve those.
+  which can resolve those. If an internal error occurs (e.g. connection failure,
+  timeout, etc.), the status code will be set to 0.
 
   The --htx option allow to use the haproxy internal htx representation using
   the htx_dump() function, mainly used for debugging.