On the configuration reference page: http://docs.basho.com/riak/kv/2.2.0/configuring/reference/
The search.anti_entropy.throttle.$tier.solrq_queue_length parameter appears to be referenced as mailbox in the description column, eg: anti_entropy.throttle.tier1.mailbox_size. The associated delay parameter appears to match.
Additionally, see the reference to mailbox in the following description column for search.anti_entropy.throttle.$tier.delay.