Better document how comparison operators work on ROW objects

The MySQL documentation explains how the comparison operators work on ROW objects. For example:

For row comparisons, (a, b) < (x, y) is equivalent to:

(a < x) OR ((a = x) AND (b < y))

https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#operator_less-than

The relevant MariaDB documentation pages do not include this information. For example:

https://mariadb.com/kb/en/library/less-than/

MDEV-16393 has a concrete example.