doc: Add description for PGSSLCRLDIR

This was missing in the section dedicated to the supported environment
variables of libpq.  Oversight in f5465fad.

Reviewed-by: Daniel Gustafsson, Kyotaro Horiguchi
Discussion: https://postgr.es/m/YLhI0mLoRkY3u4Wj@paquier.xyz

doc/src/sgml/libpq.sgml

@@ -7836,6 +7836,16 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
      </para>
     </listitem>
 
+    <listitem>
+     <para>
+      <indexterm>
+       <primary><envar>PGSSLCRLDIR</envar></primary>
+      </indexterm>
+      <envar>PGSSLCRLDIR</envar> behaves the same as the <xref
+      linkend="libpq-connect-sslcrldir"/> connection parameter.
+     </para>
+    </listitem>
+
     <listitem>
      <para>
       <indexterm>