docs: comment why two NRDOT agents are kept

Agent types `io.opentelemetry.collector` and `com.newrelic.opentelemetry.collector` should be the same except for the name

Author: DavSanchez

agent-control/agent-type-registry/newrelic/io.opentelemetry.collector-0.1.0.yaml

@@ -1,3 +1,6 @@
+# This is the same agent type as `com.newrelic.opentelemetry.collector`, kept in the repository
+# for compatibility with existing deployments and Fleet Control support.
+# It should not drift from the `com.newrelic.opentelemetry.collector` definition!
 namespace: newrelic
 name: io.opentelemetry.collector
 version: 0.1.0