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_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5d20643532cafd719482119d250c919edcac93b7

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

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