Fix a mailbox threads race, document another threads race
Fix a mailbox threads race, document another threads race

The mailbox threads race was improved by suggestion of nine++.
TASK_recv_block_CLEAR(partner) writes concurrent with a read in !TASK_in_preempt_TEST(SELF).
To repro: m parrot && ~/bin/tsan-r4356-amd64-linux-self-contained.sh ./parrot t/pmc/task.t

FROGGS/parrot,youprofit/parrot,FROGGS/parrot,tkob/parrot,FROGGS/parrot,tkob/parrot,tkob/parrot,parrot/parrot,FROGGS/parrot,tkob/parrot,tkob/parrot,youprofit/parrot,youprofit/parrot,youprofit/parrot,FROGGS/parrot,tkob/parrot,parrot/parrot,youprofit/parrot,FROGGS/parrot,FROGGS/parrot,youprofit/parrot,tkob/parrot,FROGGS/parrot,tkob/parrot,youprofit/parrot,youprofit/parrot,parrot/parrot,parrot/parrot,parrot/parrot