Document errhidecontext() where it ought to be documented.

Seems to have been missed when this function was added.  Noted while
looking at David Steele's proposal to add another similar function.

doc/src/sgml/sources.sgml

@@ -353,6 +353,15 @@ ereport(ERROR,
      includes the current statement already.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     <function>errhidecontext(bool hide_ctx)</function> can be called to
+     specify suppression of the <literal>CONTEXT:</> portion of a message in
+     the postmaster log.  This should only be used for verbose debugging
+     messages where the repeated inclusion of context would bloat the log
+     volume too much.
+    </para>
+   </listitem>
   </itemizedlist>
    </para>