Improve covering index documentation

Add missed description of pg_constraint.conincluding

Shinoda, Noriyoshi and Alexander Korotkov

doc/src/sgml/catalogs.sgml

@@ -2372,6 +2372,14 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        triggers), list of the constrained columns</entry>
      </row>
 
+     <row>
+      <entry><structfield>conincluding</structfield></entry>
+      <entry><type>int2[]</type></entry>
+      <entry><literal><link linkend="catalog-pg-attribute"><structname>pg_attribute</structname></link>.attnum</literal></entry>
+      <entry>List of the non-constrained columns which are included into
+        the same index as the constrained columns</entry>
+     </row>
+
      <row>
       <entry><structfield>confkey</structfield></entry>
       <entry><type>int2[]</type></entry>