Explain that red border depends on exceeding limit

[EoinShaughnessy]

Fixes #2140.

The character count input only shows a red border when the character limit is exceeded. If there's been some other error, and the content is within the limit, it doesn't show this border.

Some users have reported being confused by this, so we're adding guidance to explain why.

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[EoinShaughnessy]

This is how I originally reworded it:

The input shows a red border only when the user tries sending too many characters. It does not show this border when there's been an error, but the number of characters is within the limit. We felt it might cause the user difficulty if the border disappeared once they started typing into the input.

[calvin-lau-sig7]

Thanks all! Minor suggestion otherwise looks good, we'll need to check how to release this alongside #2181?

[EoinShaughnessy]

@calvin-lau-sig7 My bad! Sorry, should have double checked. If it helps, feel free to take the content out of this PR and add it to the bigger update.

[EoinShaughnessy]

@calvin-lau-sig7 The major character count update is now live 🎉 so I've tried rebasing this PR, and hopefully this little update is now also good to go...