Default check levels aren't documented precisely

- The official doc on compilation levels doesn't explain that most checks get enabled in the ADVANCED compilation mode only: https://developers.google.com/closure/compiler/docs/compilation_levels
- The Wiki page on compiler warnings misguides that the "checkTypes" check is on by default, meanwhile it's actually not when the SIMPLE compilation mode is used: https://github.com/google/closure-compiler/wiki/Warnings