[python/en] Fix 80 col limit and clarify a few points.

I noticed a few missing features, some potentially misleading information, and some lines that were longer than 80 characters. This patch is my suggestion for fixes to these, although I'm happy to update anything which seems incorrect.

I'll add some comments to the diff.
I'm not sure about this comment about underscores. The fact that it's included may make the more junior Python learners think there is something special about underscores. Really the point is that we just want to choose a new name.