Add more document for chart.show()

[jonmmease]

#3379 added a new implementation of chart.show() that invokes the active renderer as a side effect. With the "browser" renderer, this will open the chart in the browser. For notebook-based renderers, using chart.show() it's possible to display multiple charts in a single output cell, and it would be useful to explain this somewhere in the docs.

See #3379 (review)