ORDER BY spec could make more clear behavior with DESC

[alancai98]

The PartiQL spec section 12.2 currently only defines the order-by-less-than function (used for ASC) and does not explicitly mention the DESC behavior. This can be confusing for implementers as experienced in the Kotlin implementation of ORDER BY partiql/partiql-lang-kotlin#554. The behavior with respect to DESC comparison between different data types could be made clearer (whether it should compare different data types using the order-by-less-than function or the reverse/converse).

The SQL-92 spec (page 373) makes this distinction for DESC more clear:

a) Each <sort specification> specifies the sort direction for the corresponding sort key Ki. If DESC is not specified in the i-th <sort specification>, then the sort direction for Ki is ascending and the applicable <comp op> is the <less than operator>. Otherwise, the sort direction for Ki is descending and the applicable <comp op> is the <greater than operator>.

Similarly this distinction is made more clear in Couchbase's N1QL online reference: https://docs.couchbase.com/server/current/n1ql/n1ql-language-reference/orderby.html