[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.
Tuples below indicate the TypeError if you try to assign to an index, but the assignment to index operation is never introduced for lists.