[AMQ] Document AMQ_MAX_CONNECTIONS and AMQ_FRAME_SIZE variables

A-MQ recognizes AMQ_MAX_CONNECTIONS and AMQ_FRAME_SIZE environment variables when set:

• https://github.com/jboss-openshift/cct_module/blob/master/os-amq-launch/added/configure.sh#L138
• https://github.com/jboss-openshift/cct_module/blob/master/os-amq-launch/added/configure.sh#L139
• https://github.com/jboss-openshift/cct_module/blob/master/os-amq-launch/added/configure.sh#L146
• https://github.com/jboss-openshift/cct_module/blob/master/os-amq-launch/added/configure.sh#L148

But these variables aren't so far documented as available to configure / to be set:

• https://access.redhat.com/documentation/en-us/red_hat_jboss_a-mq/6.3/html-single/red_hat_jboss_a-mq_for_openshift/
• https://access.redhat.com/documentation/en-us/red_hat_jboss_a-mq/6.2/html-single/red_hat_jboss_a-mq_for_openshift/