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

[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?

Originally posted by @ydmitry in #31757 (comment)

[ffoodd]

I guess it's related to the pseudo-classes, which used to be generated even when it wasn't valid or invalid (resulting in invalid selectors). It's been fixed in #28447 1.5 years ago — and now outputs .is-#{$state} instead of ``:#{$state}`.

So the sentence you're pointing can be safely removed IMHO.