doc tweaks to CASK_LANGUAGE_REFERENCE.md

- include OS X 10.10
- include the word "specification"
- clarify how to call out of the `caveats` mini-DSL

References: #4921, though `version` is being added to the
`caveats` mini-DSL
Merge pull request #4926 from rolandwalker/doc_lang_ref_tweaks

doc tweaks to CASK_LANGUAGE_REFERENCE.md