Provide information about copying encrypted database

This is originated from https://issues.couchbase.com/browse/CBL-2319.

We should provide an info to explain that the Database.copy(fromPath, toName, config) method will copy the database without changing the encryption key. The encryption key specified in the config is the encryption key used for both the original database and copied database. If the encryption key needs to be changed or set to the copied database, the Database's changeEncryptionKey(key) will need to be call after.