pgsql: Clarify type resolution behavior for domain types.

Clarify type resolution behavior for domain types.

The user documentation was vague and not entirely accurate about how
we treat domain inputs for ambiguous operators/functions. Clarify
that, and add an example and some commentary. Per a recent question
from Adam Mackler.

It's acted like this ever since we added domains, so back-patch
to all supported branches.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/83412c7b9f2e7d159e2eebaf4da3272a603c2595

Modified Files
--------------
doc/src/sgml/typeconv.sgml | 101 ++++++++++++++++++++++++++++++++++++++------
1 file changed, 88 insertions(+), 13 deletions(-)

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