DSL: `depends_on :macos` functionality/tests/doc

* Fill in the functionality for the `depends_on :macos` stub
* de-document `caveats` method `os_version_only`
* clean up some related docs re: version strings
* add `depends_on_test.rb`
* clarify some related `depends_on :formula` doc, tests, and code
* formatting in test Casks
Merge pull request #7696 from rolandwalker/macos_dependency

DSL: `depends_on :macos`