Documentation for JMS client 'capacity' address string option

QPID-3679 originally called for documentation of 'capacity' in addition to corrections/expansion about prefetch. A patch was submitted for prefetch, so the work for 'capacity' is being split out into this JIRA.

Relevant snippet from QPID-3679:

(2) there is no mention of the capacity option in the address (which admittedly is only supported by the JMS client ta this point)