cbtransfer: Add examples of couchstore-files:// as source

We document in the manual that couchstore-files:// can be used as a
source, but not in the examples - make it clearer this is possible.

Change-Id: I107e20ead121c9d92e228c27e1f1d36576de4d5e
Reviewed-on: http://review.couchbase.org/39262
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>