docs(HTMLButtonElement): clarify setCustomValidity and reportValidity only affect submit buttons

[xonaib]

Fixes #42234

setCustomValidity() and reportValidity() on HTMLButtonElement only have practical effect for type="submit" buttons. Buttons with type="reset" or type="button" are barred from constraint validation per the HTML spec: their validity state can be set internally but the UI never reflects it, and reportValidity() always returns true.

Changes:

• Added a [!NOTE] to the setCustomValidity() page explaining the submit-only constraint.
• Added a [!NOTE] to the reportValidity() page explaining it always returns true for non-submit buttons.
• Both notes link to the spec definition of "barred from constraint validation"; the setCustomValidity() note also links to willValidate.

Validation:

• Commit hooks ran successfully: URL checks, changed xrefs, front matter, markdownlint, and Prettier.
• Checked the local MDN preview server at http://localhost:5042/en-US/docs/Web/API/HTMLButtonElement/setCustomValidity and http://localhost:5042/en-US/docs/Web/API/HTMLButtonElement/reportValidity; both pages loaded and rendered the new note content.

[github-actions]

Preview URLs (2 pages)

• /en-US/docs/Web/API/HTMLButtonElement/reportValidity
• /en-US/docs/Web/API/HTMLButtonElement/setCustomValidity

Flaws (1)

Note! 1 document with no flaws that don't need to be listed. 🎉

Found an unexpected or unresolvable flaw? Please report it here.

URL: /en-US/docs/Web/API/HTMLButtonElement/reportValidity
Title: HTMLButtonElement: reportValidity() method
Flaw count: 1

• macros:
  • Macro domxref produces link /en-US/docs/Web/API/HTMLElement/invalid_event which is a redirect

External URLs (2)

URL: /en-US/docs/Web/API/HTMLButtonElement/reportValidity
Title: HTMLButtonElement: reportValidity() method

• https://html.spec.whatwg.org/multipage/form-control-infrastructure.html (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Web/API/HTMLButtonElement/setCustomValidity
Title: HTMLButtonElement: setCustomValidity() method

• https://html.spec.whatwg.org/multipage/form-control-infrastructure.html (1 time) (Note! This may be a new URL 👀)

[pepelsbey]

Hey! Thank you for the PR. It makes it clearer, but I would suggest a few improvements:

1. Align with the pattern used on sibling pages

   The issue itself points to HTMLFieldSetElement.reportValidity() as a model, and HTMLButtonElement.checkValidity() handles the same situation the same way: both pages bake the "always returns true / no effect" behavior directly into the opening description rather than using a [!NOTE] callout.

2. Stick to the formatting used in the surrounding text: don’t split lines mid-sentences.