doc: CREATE DATABASE doesn't copy db-level perms. from template

Reported-by: david@kapitaltrading.com

Discussion: https://postgr.es/m/166007719137.995877.13951579839074751714@wrigleys.postgresql.org

Backpatch-through: 11

doc/src/sgml/manage-ag.sgml

@@ -207,6 +207,12 @@ createdb -O <replaceable>rolename</replaceable> <replaceable>dbname</replaceable
    action being taken when those databases are created.
   </para>
 
+  <para>
+   However, <command>CREATE DATABASE</command> does not copy database-level
+   <command>GRANT</command> permissions attached to the source database.
+   The new database has default database-level permissions.
+  </para>
+
   <para>
    There is a second standard system database named
    <literal>template0</literal>.<indexterm><primary>template0</primary></indexterm> This