At least one listener must be attached to a Replicator before `start()`

Right here:
https://docs.couchbase.com/couchbase-lite/2.7/java-android.html#replicator-notifications-on-a-custom-executor

We need to add a `Note:`. It should say something like this:

Note: Unless there was at least one listener registered to listen to a replicator, at the time of the most recent call to `start(...)` no subsequently registers listeners will receive notifications.