Clarify the meaning of "third party" in Third Party Library Usage

The [Third Party Library Usage](https://extensionworkshop.com/documentation/publish/third-party-library-usage/) documentation does not currently explain what Mozilla means by the term "third party". This [has crated some confusion](https://discourse.mozilla.org/t/source-code-policy/144640/4) for the React development team as React Developer Tools depends on React. 