followups on `depends_on` `:macos`/`:arch`

 * always coerce `:macos` value to array; simplify code
 * remove warning comments about unset values
 * doc explicitly that requirements are covered as well as dependencies
 * doc that `depends_on` is not consulted until `install` time
 * in docs and code, say "release" instead of "version" for the :macos release
   * todo: provide our own variable styled "release" to use in Casks, instead of Homebrew's `MacOS.Version`
 * doc that symbols are preferred for `:macos` over strings in Casks (interally, all are converted to idential `Gem::Version` objects) refs: https://github.com/caskroom/homebrew-cask/pull/7760#discussion_r21234042
 * doc: remove `os_version_only` from `cask_language_deltas.md`
 * doc: improve internal links
 * doc: recast OS X release symbol list as table
 * doc: (incidental) regularize orthography of Formula and Cask