doc: update Alfred Integration section in USAGE.md

We have two pending PRs for this section (which had a transposed line), so I'd prefer to proceed by accepting those PRs, but both proposals either remove valid content or don't incorporate suggestions from discussion.

I also checked the remainder of the docs for first-person usages, but this was the only one.

* fix transposed line
* recast first-person phrasing
* better explain what a "scope" is
* better explain what the "Caskroom" is

closes #7759
closes #6059
Sure! With one qualification, that I am changing "Caskroom" to "staging directory" or similar whenever I encounter it (the process is not complete).