pgsql: Doc: add example of type resolution in nested UNIONs.

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

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/356f85f95cb0b0e99ce51932c2a9dbd292f0b292

Modified Files
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doc/src/sgml/typeconv.sgml | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)