Highlevel docs for time_units

[benjeffery]

We need to explain what it means and how to use it.

[hyanwong]

At the moment, the only uses that I know of are

1. If set to "uncalibrated" (tskit.TIME_UNITS_UNCALIBRATED) we don't allow branch-length stats to be calculated
2. The time units are displayed on the Y axis when plotting SVG trees.