docs - add string_agg function as a migration issue
aggregate in a previous Greenplum release that called the built-in
<codeph>int4_avg_accum()</codeph> function, you will need to revise your aggregate for
the new signature. </li>
+ <li>The <codeph>string_agg(expression)</codeph> function has been removed from Greenplum 6.
+ The function concatenates text values into a string. You can replace the single argument
+ function with the function <codeph>string_agg(expression, delimiter)</codeph> and specify
+ an empty string as the <codeph>delimiter</codeph>, for example
+ <codeph>string_agg(txt_col1, '').</codeph></li>
<li><codeph>gpbackup</codeph> saves the distribution policy and distribution key for each
table in the backup so that data can be restored to the same segment. If a table's
distribution key in the Greenplum 4.3 or 5 database is incompatible with Greenplum 6,
