Improve Proc.call documentation

* Clarify variable names
* Add markup around inline example
* Reordered examples to match descriptions
Having the submitter fix everything they possibly could around the issue they are interested in leads to frustration for the submitter.  It also dilutes the value of the pull request as a unit-of-work (that it should do only one thing).

As a general rule I prefer shorter words to longer words if they have the equivalent meaning.  In this case, the OS X dictionary says for syntactic: "of or according to syntax" while for syntax: "the arrangement of words and phrases to create well-formed sentences in a language" and "a set of rules for analysis of this".

Thus I prefer "syntax" since it gives the reader a better idea of what the word means when they look it up in the dictionary.

Related to your google search, there's also [Google's Ngram viewer](https://books.google.com/ngrams/graph?content=syntax%2Csyntactic&case_insensitive=on&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t4%3B%2Csyntax%3B%2Cc0%3B%2Cs0%3B%3Bsyntax%3B%2Cc0%3B%3BSyntax%3B%2Cc0%3B%3BSYNTAX%3B%2Cc0%3B.t4%3B%2Csyntactic%3B%2Cc0%3B%2Cs0%3B%3Bsyntactic%3B%2Cc0%3B%3BSyntactic%3B%2Cc0) which shows "syntax" being much more common than "syntactic".

Granted, "syntactic sugar" is much more common in these books, but I imagine books with "syntactic sugar" are primarily academic given the frequency count compared to just "syntax".

I prefer the language of Ruby's documentation to sacrifice correctness in favor of ease of understanding by readers without a high level of English fluency.