Improve Advanced Usage examples

[lrq3000]

Description:
A few improvement of Advanced Usage examples:

• Rework outputs python-version to replace missing words in description and clarify example.
• Mention '*' for python-version to get the latest stable python release.
• Mention that if statements need to use single-quotes, not double-quotes (and not no-quotes) to test python-version against string literals.

I tried to keep with the conventions used elsewhere in the document, and for my contributions to be as concise as possible (eg, the if statement is not its own paragraph but is just one additional line in the Matrix Testing example, the most relevant one as it's the most common case scenario for an if testing).

Related issue:
Closes #644.

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.