updated gpconfig reference with info about setting GUC value to empty string.
mirrors, the master, and the standby master.</pd>
<pd>The utility correctly quotes the value when adding the setting to the
<codeph>postgresql.conf</codeph> files.</pd>
+ <pd>To set the value to an empty string, enter empty single quotes (<codeph>''</codeph>). </pd>
</plentry>
<plentry>
<pt>-m | --mastervalue <varname>master_value</varname></pt>
