docs(validation): add validateBeforeSave and invalidate

[dandv]

Add validateBeforeSave and invalidate() to the Validation guide.

Is there a way to mark a field as valid, so it will be skipped by the pre-save validation?

[vkarpov15]

Thanks 👍

[vkarpov15]

Re: "Is there a way to mark a field as valid, so it will be skipped by the pre-save validation?", Document#$markValid() is the inverse of invalidate().