add 'milliseconds' to duration javadoc

[aSemy]

I haven't checked if it is actually milliseconds! See #73 for more info.

Resolve #73

TODO

• confirm that duration is in milliseconds

[tresf]

#73

The unix source code calls a second "duration / 1000", then converts to nanoseconds calling nanosleep.

jssc/src/cpp/_nix_based/jssc.cpp

Line 635 in f556486

int sec = (duration >= 1000 ? duration/1000 : 0);

The windows source code calls Sleep(), which is nanoseconds.

Confirmed. :)