Doc: add example of type resolution in nested UNIONs.

Section 10.5 didn't say explicitly that multiple UNIONs are resolved
pairwise.  Since the resolution algorithm is described as taking any
number of inputs, readers might well think that a query like
"select x union select y union select z" would be resolved by
considering x, y, and z in one resolution step.  But that's not what
happens (and I think that behavior is per SQL spec).  Add an example
clarifying this point.

Per bug #15129 from Philippe Beaudoin.

Discussion: https://postgr.es/m/152196085023.32649.9916472370480121694@wrigleys.postgresql.org

doc/src/sgml/typeconv.sgml

@@ -1077,5 +1077,34 @@ result type is resolved as <type>real</>.
 </para>
 </example>
 
+<example>
+<title>Type Resolution in a Nested Union</title>
+
+<para>
+<screen>
+SELECT NULL UNION SELECT NULL UNION SELECT 1;
+
+ERROR:  UNION types text and integer cannot be matched
+</screen>
+This failure occurs because <productname>PostgreSQL</productname> treats
+multiple <literal>UNION</literal>s as a nest of pairwise operations;
+that is, this input is the same as
+<screen>
+(SELECT NULL UNION SELECT NULL) UNION SELECT 1;
+</screen>
+The inner <literal>UNION</literal> is resolved as emitting
+type <type>text</type>, according to the rules given above.  Then the
+outer <literal>UNION</literal> has inputs of types <type>text</type>
+and <type>integer</type>, leading to the observed error.  The problem
+can be fixed by ensuring that the leftmost <literal>UNION</literal>
+has at least one input of the desired result type.
+</para>
+
+<para>
+<literal>INTERSECT</literal> and <literal>EXCEPT</literal> operations are
+likewise resolved pairwise.  However, the other constructs described in this
+section consider all of their inputs in one resolution step.
+</para>
+</example>
 </sect1>
 </chapter>