ALTER SUBSCRIPTION / REFRESH docs: explain copy_data

The docs are ambiguous as to which tables would be copied over when the
copy_data parameter is true in ALTER SUBSCRIPTION ... REFRESH PUBLICATION.
Make it clear that it only applies to tables which are new in the
publication.

Author: David Christensen (reword by Álvaro Herrera)
Discussion: https://postgr.es/m/95339420-7F09-4F8C-ACC0-8F1CFAAD9CD7@endpoint.com

doc/src/sgml/ref/alter_subscription.sgml

@@ -128,7 +128,8 @@ ALTER SUBSCRIPTION <replaceable class="parameter">name</replaceable> RENAME TO <
          <para>
           Specifies whether the existing data in the publications that are
           being subscribed to should be copied once the replication starts.
-          The default is <literal>true</literal>.
+          The default is <literal>true</literal>.  (Previously subscribed
+          tables are not copied.)
          </para>
         </listitem>
        </varlistentry>