Class method which returns an SQL WHERE fragment to search for parameters
specified in HASHREF. Accepts the following parameters for
-L<FS::cust_bill::search_sql_where>: C<_date>, C<invnum_min>, C<invnum_max>,
+L<FS::cust_bill/search_sql_where>: C<_date>, C<invnum_min>, C<invnum_max>,
C<agentnum>, C<custnum>, C<cust_classnum>, C<refnum>, C<payby>. Also
accepts the following: