docs(ngModel): clarify usage of formatters / parsers and $setValidity
docs(guide/forms, ngModel): update list of css classes
docs(input): clarify input and ngModel behavior
docs(guide/forms): update custom validation section with $validators
docs(guide/forms, input): add information how to modify built-in validators

Closes #5757
Closes #7798
docs(guide/form): shorten line lengths for better readability