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_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b666c55b2caee0e0c1f9afc404dd00f79c98bc3d

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