Doc examples with zmq_msg_close usage - parameter is 'zmq_msg_t *'

Documentation examples for zmq_msg_get and zmq_msg_more functions have an
incorrect call to zmq_msg_close function - with 'zmq_msg_t' as a parameter
despite 'zmq_msg_t *' is required, so it is impossible to compile these
examples properly.

Also for zmq_msg_get example - declaration of zmq_msg_t variable is added
(like it is done in other examples).
Merge pull request #733 from mingulov/doc

Doc examples with zmq_msg_close usage - parameter is 'zmq_msg_t *'