[Docs] Clarify suggested use of custom validation styles, add note about...

[Docs] Clarify suggested use of custom validation styles, add note about resetting form appearance after validation (#25628)

* Expand/clarify recommendation for using custom validation styles

Seems support for correctly announcing browser default navigation bubbles/notifications has improved recently. Now it seems only Chrome (at least on Win and Android, not tested on macOS) is the odd one out.

However, we should really make sure the custom validation messages are also tied correctly to their relevant form controls - need to dig out the PR that will ensure that.

* Add note about resetting form appearance after submission

xref https://github.com/twbs/bootstrap/issues/25626

docs/4.0/components/forms.md

@@ -700,7 +700,7 @@ While Bootstrap will apply these styles in all browsers, Internet Explorer 11 an
 Provide valuable, actionable feedback to your users with HTML5 form validation–[available in all our supported browsers](https://caniuse.com/#feat=form-validation). Choose from the browser default validation feedback, or implement custom messages with our built-in classes and starter JavaScript.
 
 {% callout warning %}
-We **highly recommend** custom validation styles as native browser defaults are not announced to screen readers.
+We currently recommend using custom validation styles, as native browser default validation messages are not consistently exposed to assistive technologies in all browsers (most notably, Chrome on desktop and mobile).
 {% endcallout %}
 
 ### How it works
@@ -709,6 +709,7 @@ Here's how form validation works with Bootstrap:
 
 - HTML form validation is applied via CSS's two pseudo-classes, `:invalid` and `:valid`. It applies to `<input>`, `<select>`, and `<textarea>` elements.
 - Bootstrap scopes the `:invalid` and `:valid` styles to parent `.was-validated` class, usually applied to the `<form>`. Otherwise, any required field without a value shows up as invalid on page load. This way, you may choose when to activate them (typically after form submission is attempted).
+- To reset the appearance of the form (for instance, in the case of dynamic form submissions using AJAX), remove the `.was-validated` class from the `<form>` again after submission.
 - As a fallback, `.is-invalid` and `.is-valid` classes may be used instead of the pseudo-classes for [server side validation](#server-side). They do not require a `.was-validated` parent class.
 - Due to constraints in how CSS works, we cannot (at present) apply styles to a `<label>` that comes before a form control in the DOM without the help of custom JavaScript.
 - All modern browsers support the [constraint validation API](https://www.w3.org/TR/html5/sec-forms.html#the-constraint-validation-api), a series of JavaScript methods for validating form controls.