Updated docs for new methods and clarifications
Updated docs for new methods and clarifications

- Updated docs for how to properly hook `validate_and_save` into
  WordPress
- Adds documentation for how `link` and `multiselect` fields work

Scotchester/cms-toolkit,kurtw/cms-toolkit,cfpb/cms-toolkit,Scotchester/cms-toolkit,kurtw/cms-toolkit,cfpb/cms-toolkit