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_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/f8ab54b7ecc73d05dec3a6a620adc924b5e5ca3f

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

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