docs: (Client Action API) mention user should pre-exist when creating action

[ShubhamPalriwala]

What does this PR do?

On running the endpoint with a random userId, we get a 500 error without any helpful error message. Until we handle this error message & status code, it is helpful to mention this in the docs to the user

How should this be tested?

• Test A
• Test B

Checklist

Required

• Filled out the "How to test" section in this PR
• Read How we Code at Formbricks
• Self-reviewed my own code
• Commented on my code in hard-to-understand bits
• Ran pnpm build
• Checked for warnings, there are none
• Removed all console.logs
• Merged the latest changes from main onto my branch with git pull origin main
• My changes don't cause any responsiveness issues
• First PR at Formbricks? Please sign the CLA! Without it we wont be able to merge it 🙏

Appreciated

• If a UI change was made: Added a screen recording or screenshots to this PR
• Updated the Formbricks Docs if changes were necessary

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2 Ignored Deployments

Name	Status	Preview	Updated (UTC)
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[github-actions]

Thank you for following the naming conventions for pull request titles! 🙏

[mattinannt]

@ShubhamPalriwala 👍 thanks for the docs update :-)