improve(cockpit/plugins): minor improvement and documentation
improve(cockpit/plugins): minor improvement and documentation

Add some information about the plugin exclusion mechanism and move the
exclusion collection out from the provider implementation.

ingorichtsmeier/camunda-bpm-platform,camunda/camunda-bpm-platform,camunda/camunda-bpm-platform,langfr/camunda-bpm-platform,langfr/camunda-bpm-platform,langfr/camunda-bpm-platform,camunda/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,camunda/camunda-bpm-platform,langfr/camunda-bpm-platform,langfr/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,langfr/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,camunda/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,camunda/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform