MCO-511 Add documentation for ssl.ciphers connector options

Here we add documentation of the new
plugin.{rabbitmq,activemq}.pool.$n.ssl.ciphers options, added in
2.6.1.

As part of that we're adding a full parameter reference to the
reference/connector_activemq and reference/connector_rabbitmq pages
to make them more complete reference pages.
Merge pull request #261 from richardc/MCO-511-update_documentation_2.6.1

MCO-511 Add documentation for ssl.ciphers connector options