Doc: Add output to example code in programming FAQ (GH-21346)

Add output hint to document, part faq/programming, section [How do I write a function with output parameters (call by reference)?](https://docs.python.org/3/faq/programming.html#how-do-i-write-a-function-with-output-parameters-call-by-reference).

This patch make the output hint just like prefix code block.