docs: improve form guide consistency and validation examples (#90361)
### What?
Small improvements to the Forms guide for consistency and clarity.
### Why?
Some examples didn’t fully match the recommendations in the guide (e.g.
HTML validation), and there was a minor inconsistency between
`event.target` and `event.currentTarget`. This update keeps the examples
aligned and easier to follow.
### How?
- Added `required` to relevant input examples
- Standardized usage of `event.currentTarget`
- Made minor wording refinements for clarity
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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>