Warn that kafka-connect group.id must not conflict with connector names

If the group.id value happens to have the same value as a connector names the following error will be issued:

Attempt to join group connect-elasticsearch-indexer failed due to: The group member's supported protocols are incompatible with those of existing members.

Maybe the documentation for Distributed Worker Configuration group.id could be worded:

A unique string that identifies the Connect cluster group this worker belongs to. This value must be different than all connector configuration 'name' properties.