Improve compact_prop and chain_prop docstring in plot_trace

[OriolAbril]

The description is outdated, the parameters are no longer a tuple and they are missing the default values. Link to doc page: https://python.arviz.org/en/latest/api/generated/arviz.plot_trace.html.

Default is color to identify chain if compact=False, otherwise color identifies compact and linestyle identifies chain.

I will fix this, please don't work on this.