docs: Review necessarity of warning about customizing values form-valiation-state mixin

Created by: ydmitry

Wanted to ask about phrase in the docs in /site/content/docs/5.0/forms/validation.md:

Please note that we do not recommend customizing these values without also modifying the `form-validation-state` mixin.

Should we keep this? Why need to modify a mixin? Seems should be OK now without modifying. What do you think?