chore(REST): add docs about REST API extensions

related to CAM-2190, CAM-2191, CAM-2195
chore(REST): add docs how to deliver a message to multiple executions

related to CAM-2006
fix(Migration Guide): Version name DB upgrade

related to #CAM-2416
chore(REST): add docs about REST API extensions

related to CAM-2197
fix(bpmn references): proofread and correct

related to #CAM-2375
fix(userguide): proofread and correct

related to #CAM-2329 and #CAM-2326
chore(REST): add docs how to deliver a message to multiple executions

related to #CAM-2006