Certain cautions should be documented for database.close()

The `IllegalStateException: Attempt to perform an operation on a closed database.` exception would likely generate if the user closed the database immediately after starting the replicator:

replicator.start();

database.close();

Although this use case is very unlikely to occur, we should document why this exception is thrown and how can the user troubleshoot this scenario and fix it. Few places where we might want to consider documenting this are:

https://docs.couchbase.com/couchbase-lite/current/java/advance/java-p2psync-websocket-using-active.html#configure-replicator
https://docs.couchbase.com/couchbase-lite/current/android/product/java-android-releasenotes.html
http://docs.couchbase.com/mobile/2.8.0/couchbase-lite-android/index.html?com/couchbase/lite/Database.html#close--