Add documentation of maps in supervisor flags and child specs
Update dialyzer test with maps in supervisor properties

After introducing maps for supervisor flags and child specs two tests
in behaviour_SUITE started failing:

behaviour_SUITE:custom_sup - failed because the type of the Period
(MaxT) property had been changed (corrected) from non_neg_integer() to
pos_integer() in the specification of the init callback in
supervisor.erl

behaviour_SUITE:supervisor_incorrect_return - same as above, plus the
introduction of #{} as possible value for the supervisor flags and
child_specs.