reactor: document intentional bitwise-on-bool op in smp_pollfn::poll()
reactor: document intentional bitwise-on-bool op in smp_pollfn::poll()

We intentionally reduce two booleans with `|` instead of `||` since
we want the side effects of the second expression to take effect
unconditionally.

Document that, and document it to gcc 12's new warning as well by
casting the operands to int, as the warning text recommends.
Message-Id: <0e2b99f9a135cee46c39d944dcc7b7190a9f5cdb@scylladb.com>

scylladb/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar