Improve Proc.call documentation

* Clarify variable names
* Add markup around inline example
* Reordered examples to match descriptions
All right, to be honest I'm quite new to the Ruby's world, and therefore to everything about contribution rules. I though this PR would be the occasion to correct it.

Regarding _syntax sugar_ vs _syntactic sugar_, it's just that statistically I heard far more the second one, and before posting the comment I checked the first expression by searching for it on Google first. I'm not saying this is a reference, but it was returning a lot of results for _syntactic sugar_ instead, including the Wikipedia article named the same way. Directly searching for _syntactic sugar_ also gave a lot more results.

But I'm not a native English speaker, and I wouldn't fight against idioms.