iris: Explain how conditional aux accesses work

iris: Explain how conditional aux accesses work

Apart from an issue with fast clears that will be addressed soon,
aux-state tracking with conditional rendering works because the
aux-state info needed for performing required resolves is never lost.

Add comments explaining how this works. Assertions are omitted to avoid
having to pass render_condition variables into
iris_resource_prepare_access and iris_resource_finish_write.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7762>