Add link from COPY ref page to psql \copy.

Jeff Janes

doc/src/sgml/ref/copy.sgml

@@ -353,7 +353,7 @@ COPY <replaceable class="parameter">count</replaceable>
    <para>
     Do not confuse <command>COPY</command> with the
     <application>psql</application> instruction
-    <command>\copy</command>. <command>\copy</command> invokes
+    <command><link linkend="APP-PSQL-meta-commands-copy">\copy</link></command>. <command>\copy</command> invokes
     <command>COPY FROM STDIN</command> or <command>COPY TO
     STDOUT</command>, and then fetches/stores the data in a file
     accessible to the <application>psql</application> client. Thus,

doc/src/sgml/ref/psql-ref.sgml

@@ -826,7 +826,7 @@ testdb=>
         </listitem>
       </varlistentry>
 
-      <varlistentry>
+      <varlistentry id="APP-PSQL-meta-commands-copy">
         <term><literal>\copy { <replaceable class="parameter">table</replaceable> [ ( <replaceable class="parameter">column_list</replaceable> ) ] | ( <replaceable class="parameter">query</replaceable> ) }
         { <literal>from</literal> | <literal>to</literal> }
         { <replaceable class="parameter">filename</replaceable> | stdin | stdout | pstdin | pstdout }