DOC: hlua: Add a note to warn user about httpclient object reuse

DOC: hlua: Add a note to warn user about httpclient object reuse

It is not supported to reuse an lua httpclient instance to process several
requests. A new object must be created for each request. Thanks to the
previous patch ("BUG/MEDIUM: httpclient: Throw an error if an lua httpclient
instance is reused"), an error is now reported if this happens. But it is
not obvious for users. So the lua-api docuementation was updated accordingly.

This patch is related to issue #2986. It should be backported with the
commit above.

(cherry picked from commit cb7a2444d149de8e0ce89d7b4a450e5c36822412)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>