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_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6079e5d4bce4dc6c52e83d61aadb091a829a1a39

Modified Files
--------------
doc/src/sgml/typeconv.sgml | 102 ++++++++++++++++++++++++++++++++++++++------
1 file changed, 89 insertions(+), 13 deletions(-)

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