doc: Add documentation for PGLOADBALANCEHOSTS environment variable in libpq.

Commit 7f5b198 introduced the PGLOADBALANCEHOSTS environment
variable for connection load balancing in libpq. However, documentation
for this variable was missing.

Author: Fujii Masao
Reviewed-by: Jelte Fennema
Discussion: https://postgr.es/m/e2b44cf7-aa16-e726-1d4a-a69508ebf6b9@oss.nttdata.com

Author: MasaoFujii

doc/src/sgml/libpq.sgml

@@ -8307,6 +8307,16 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
       linkend="libpq-connect-target-session-attrs"/> connection parameter.
      </para>
     </listitem>
+
+    <listitem>
+     <para>
+      <indexterm>
+       <primary><envar>PGLOADBALANCEHOSTS</envar></primary>
+      </indexterm>
+      <envar>PGLOADBALANCEHOSTS</envar> behaves the same as the <xref
+      linkend="libpq-connect-load-balance-hosts"/> connection parameter.
+     </para>
+    </listitem>
    </itemizedlist>
   </para>