pgsql: Doc: clarify the default collation behavior of domains.

Doc: clarify the default collation behavior of domains.

The previous wording was "the underlying data type's default collation
is used", which is wrong or at least misleading.  The domain inherits
the base type's collation behavior, which if "default" actually can
mean that we use some non-default collation obtained from elsewhere.

Per complaint from Jian He.

Discussion: https://postgr.es/m/CACJufxHMR8_4WooDPjjvEdaxB2hQ5a49qthci8fpKP0MKemVRQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d26ac35d0d7cb42a0453e69e0e9ce6d163b55927

Modified Files
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doc/src/sgml/ref/create_domain.sgml | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)