Document that edd_ajax_checkout_errors hook clears errors on priority 10

[ericnicolaas]

The edd_ajax_checkout_errors hook in edd_process_purchase_form is used by edd_print_errors on priority 10, which clears the errors.

As a 3rd party developer, if you want to use this hook it is confusing that edd_get_errors() returns false if you hook in on or after priority 10.

I would suggest documenting this behaviour where the hook is called, and also suggest changing the priority of edd_print_errors to something higher so developers can use the default priority without missing the errors.

[cklosowski]

I think this is actually a great idea, both the documentation, and raising the priority to something higher to 10.