Clarify `elif` example

The program is now given with and without `elif`, showing what it is
equivalent to.
Correct blank lines after blocks error again
Make the first `for` example more interesting

It now prints "[ ]" before every item, suggesting a check list.
Add missing output to fourth functions example
Replace single quotes in code with double quotes

It appears that single quotes in code blocks are turned into 'smart'
quotes by LaTeX, causing problems when copy-pasted into an interpreter.
Double quotes do not suffer from this problem.
Remove redundant `list` call in counting example
Fix numerous typos and questionable wordings