docs: clarify useFormContext formState subscriptions

[hiromieguchi802-lab]

Summary

• document that formState values accessed through useFormContext() inside a FormProvider tree do not establish subscriptions on their own
• recommend useFormState when subscribing to values like errors, isDirty, and dirtyFields

Fixes #1193.

Testing

• reviewed the updated docs copy in src/content/docs/useformcontext.mdx
• verified the diff is limited to the useFormContext rules section

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[hiromieguchi802-lab]

Quick follow-up: I pushed a small formatting fix for the useFormContext note in this PR. The remaining failing Run format code check is now reporting src/content/docs/useform.mdx, which is outside this PR's change set. The other blocking status still appears to be the fork-side Vercel authorization.

[hiromieguchi802-lab]

I pushed a cleanup update and rebased this branch onto the latest master.

This also removes an unrelated change that had slipped into useform.mdx, so the PR is now limited to the useFormContext note about using useFormState when subscription behavior is needed.