[a11y] Add a note in Form validation

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

In https://ouds-main--boosted.netlify.app/docs/0.5/forms/validation/, I thinkk we must add a note:

• Due to constraints in how CSS works, we cannot (at present) apply styles to a that comes before a form control in the DOM without the help of custom JavaScript.
• All modern browsers support the constraint validation API, a series of JavaScript methods for validating form controls. **But, not all couples, user agent/screen reader, support such an API feature, so do not use it without an accessibility fallback. **
• Feedback messages may utilize the browser defaults (different for each browser, and unstylable via CSS) or our custom feedback styles with additional HTML and CSS.

Motivation and context

https://a11ysupport.io/tests/tech__aria__aria-errormessage-with-aria-invalid-true#assertion-aria-aria-errormessage_attribute-convey_pertinent

Open the forms validation documentation at https://ouds-main--boosted.netlify.app/docs/0.5/forms/validation/ and review the existing browser defaults section. Add the proposed accessibility note, preserving links to the constraint validation API and browser defaults, then check that the rendered documentation and links are correct.