Clarify global side effects in documentation

[nealmcb]

I was surprised that a statement like

ipy_table.set_column_style(1, align='right')

would re-render a table I had worked with before. How does this work? There seems to be some sort of global state that the library is maintaining, holding on to something like the most recently-generated table. In these days of object-oriented programming, functional programming and parallel processing I think maintaining state like that can often be problematic. What issues does it help with, and what issues does it cause?