Enhances pg_createsubscriber documentation for the -d option.

Hi,

Currently, pg_createsubscriber converts streaming replication to
logical replication using the specified database name. If the database
name is not provided, it is obtained from the --publisher-server
option. If the database name is not specified in either the --database
or --publisher-server option, an error is reported. This behavior is
not documented. The attached patch includes the necessary
documentation updates.

Regards,
Vignesh