[Data logging] Default in API docs for log.set_labels timestamp parameter should be SECONDS not MILLISECONDS

[microbit-matt-hillsdon]

Spotted by Rob reviewing the simulator work.

The default for the log.set_labels timestamp parameter should be SECONDS not MILLISECONDS.

The behaviour is correct, just the docs/stubs that are misleading.

[microbit-matt-hillsdon]

Note docs for data logging aren't merged at the time of writing - see bbcmicrobit/micropython#720 (this may change soon).

[microbit-robert]

Our example for log.set_labels should use a keyword argument to set the timestamp. See example and conflicting docs below: