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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cc5de7ba255dad24e57fd77578a70581a17a8b2c

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