Improve Proc.call documentation

* Clarify variable names
* Add markup around inline example
* Reordered examples to match descriptions
Since the original has the same grammatical structure as this pull request I don't think there is a need to change it for this pull request about updating the examples for clarity.

Ruby documentation should be written in simple English to make it easier to understand by non-native readers. I prefer "syntax sugar" under this guideline. Perhaps those two sentences could be rewritten into a simpler sentence in a future pull request.