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

Author: tglsfdc

doc/src/sgml/ref/create_domain.sgml

@@ -94,7 +94,8 @@ CREATE DOMAIN <replaceable class="parameter">name</replaceable> [ AS ] <replacea
       <listitem>
        <para>
         An optional collation for the domain.  If no collation is
-        specified, the underlying data type's default collation is used.
+        specified, the domain has the same collation behavior as its
+        underlying data type.
         The underlying type must be collatable if <literal>COLLATE</literal>
         is specified.
        </para>