Clarify hx-validate documentation

[MikeMoolenaar]

Fixes #2185

Description

The hx-validate documentation says "Form elements do this by default, but other elements do not" which is quite confusing, I interpreted this to also mean <input> elements validate by default. This PR solves that confusion and also makes it clear that hx-validate must be set to "true".

This change has not been explicitly approved by the maintainers via the issue, hope that's ok. If not, please let me know and I won't make a PR next time until it's approved.

Corresponding issue: #2185

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Yup, looks good.