TS-2243 document exec_thread.affinity

document proxy.config.exec_thread.affinity, according to the code in
ocore/eventsystem/UnixEventProcessor.cc 160+

n.b.: I'm not documenting the default case, that treats every value that
is not 0, 2, or 3 as `1`. I couldn't find the words.

IMO, we should error out here, rather than potentially mis-interpreting
an admin's intent.