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
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a8c63282ddce7ab8826dfb47879c62f010b307d8

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