doc: add example of % substitution for connection URIs

Reported-by: Zhou Digoal

Discussion: https://postgr.es/m/20170912133722.25637.91@wrigleys.postgresql.org

Backpatch-through: 10

doc/src/sgml/libpq.sgml

@@ -840,7 +840,9 @@ postgresql:///mydb?host=localhost&port=5433
 
    <para>
     Percent-encoding may be used to include symbols with special meaning in any
-    of the <acronym>URI</acronym> parts.
+    of the <acronym>URI</acronym> parts, e.g. replace <literal>=</> with
+    <literal>%3D</>.
+
    </para>
 
    <para>