docs(textarea): add corrected guidelines link and rootClass

[marissahuysentruyt]

• adds a spectrum-Textarea rootClass to textarea component. It doesn't have any styles, is strictly an addition and doesn't overwrite/interfere with the textfield component in the text area template.
• adds an additional spectrum object to contain the text area guidance link

Description

How and where has this been tested?

Please tag yourself on the tests you've marked complete to confirm the tests have been run by someone other than the author.

Validation steps

Regression testing

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1. The documentation pages for at least two other components are still loading, including:

• [ ] The pages render correctly, are accessible, and are responsive.

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• [ ] VRTs have been run and looked at.
• [ ] Any VRT changes have been accepted (by reviewer and/or PR author), or there are no changes.

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To-do list

• [ ] I have read the contribution guidelines.
• [ ] I have updated relevant storybook stories and templates.
• [ ] I have tested these changes in Windows High Contrast mode.
• [ ] If my change impacts other components, I have tested to make sure they don't break.
• [ ] If my change impacts documentation, I have updated the documentation accordingly.
• [ ] ✨ This pull request is ready to merge. ✨

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Latest commit: 64192ffff30ced56a822d1400d0dd23c3960d572

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Total size: 4.26 MB^*

🎉 No changes detected in any packages

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[castastrophe]

@marissahuysentruyt I wasn't sure about this test case, is this still accurate with the --sizeM at the end?

When inspecting any of the text area templates, the additional spectrum-Textfield--multiline--sizeM class has been added to the parent container. (this has no other styles, besides the styles for spectrum-Textfield--multiline)