Clarify use of database.copy() in prebuilt database topic

Need to clarify that copy does just that. It doesn't move the database. So need to manually remove the original version to release space.

Also, may need to include a trailing directory separator in the source database path on some platforms .

Per forum discussion https://forums.couchbase.com/t/what-is-the-proper-way-to-install-a-pre-built-database/30357/24 involving Blake Meike