doc(extension method): revise examples

[robstoll]

On purpose not in #8507 as it contains more changes than usual and the chances are higher that we need to discuss them.

[odersky]

Looks good now. It's great to see these improvements!

[robstoll]

@odersky I still have to fix the mistake with bringing divide into scope (which is not even an extension method). So please don't merge yet. I thought I wait until I get a feedback from you (or any other) concerning the simple name rule which seems to be a simplification in my point of view. IMO we don't need to be very specific in the docs about this minor detail as people using simple names won't have problems. However, I would suggest that we state something along the line of Using a qualified name for the receiver of an extension method might work for certain cases but should be seen as experimental and could change at any time. What do you think?

Edit not necessary, I confused myself, the under a simple name is actually fine. I am going to add my insights to the documentation and do the remaining improvements.

[robstoll]

Did the last adjustments, good to merge IMO

[odersky]

Thank you for the contribution!