Revise passing state documentation

[justinlevi]

Update the alternative approach to using returned results to be more inline with the suggestions proposed here:
#382

Using the Symfony Process component for returning simple system info.

Overview

This pull request:

Attempts to improve the documentation a little.

[greg-1-anderson]

The focus of that section of the documentation is how to pass information from one task to another. The example of using PHP's exec is just a side point that one should only use defer for complex task collections. It's off-topic to diverge into a discussion on how to call the Symfony Process component here.

Optionally, if you wanted to put that example somewhere else, then we could include a link to it from this section.

If you do move this addition, I think you can use $process->run() instead of start() + wait(), and you are missing a $ on the last line of the code example.