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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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e0047269a8c92e5af30eaa2c587e60ff21ec64fb

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