Metadata / DOC improvements

- Moved 'how it works' out of DOC block (this is for user documentation, rather than maintainer/implementation documentation – I've also clarified this on [the wiki](https://github.com/Modernizr/Modernizr/wiki/Authoring-a-v3-test))
- Turned `"authors"` and `"warnings"` fields into arrays
- Corrected `"property"` field to match name used in `Modernizr.addTest()` (these must be the same)