Perhaps listing the data types would make things easier to understand? Something like:
* Casting is supported between xml and the built-in character (character, character varying, or text), numeric, boolean, binary, and date/time types. The date/time types (date, time with time zone, timestamp with time zone, and interval) are converted to their corresponding XML Schema types: xs:date, xs:time, xs:dateTime, and xs:duration.
If we link to them, then the user will not have any doubts of what datatypes we work
+ It supports casting between <type>xml</type> and <link linkend="datatype-character-table">character</link>, <link linkend="datatype-numeric">numeric</link>, <link linkend="datatype-datetime">date/time</link>, <link linkend="datatype-boolean">boolean</link> and <link linkend="datatype-binary">binary</link> data types.
If the user gets a "ERROR: unsupported target data type for XMLCast" using a domain (being an alias for a base type or not) and the documentation clearly lists the supported types, wouldn't that be enough? If not, do you have anything in mind that we could add to the docs?
I think being explicit in the previous paragraph then is fine to not mention domains.
