Improve Proc.call documentation

* Clarify variable names
* Add markup around inline example
* Reordered examples to match descriptions
I strongly disagree about "syntax sugar".
"syntactic sugar" should be preferred in every situation because:
* There is no such thing as "syntax sugar".
* Having a simpler word for "syntactic" does not help in this case since the meaning of "sugar" is highly context-dependent here.
* There is an accurate technical word with no really simpler alternative.

However, as this is present before the patch, there is of course no need for this particular PR to fix it.
But I believe typos such as double space or "syntax sugar" can be easily fixed in such a commit, and there is no real value for a separate commit here.