and search for "dir " (with the space). There will be 43 results. Of these, only a few (9 results) have an explanation of the meaning of "newer" and "older", like this:
rvdir - In which direction to enumerate (enum) newer - List oldest first. Note: rvstart has to be before rvend. older - List newest first (default). Note: rvstart has to be later than rvend. One value: newer, older Default: older
The others are documented like so:
afldir - The direction in which to enumerate One value: newer, older Default: older
This could be improved to match the first example above.
It seems includes/api/ApiQueryBase.php has a function getDirectionDescription to help with this.