Remove some out-of-date restrictions and clarify some wording.

Inspired by #788, I've clarified some wording, and also updated the text to reflect several restrictions that have been lifted recently.  (Especially returning modified variables from lifted loops!)
I'm thinking, perhaps we could have a glossary with definitions for "nopython mode", "object mode", and others?
It should be doable with http://sphinx-doc.org/markup/para.html?highlight=glossary#directive-glossary