Update managing-users.md to include RV permission

[svalsdottir]

This article should mention that RV is available only to the Admin user.

What Changed

• Called out that Admin is the only user that can use Recipient Validation tool
• Minor grammar edits
• Made link at end of article contextual

PR Checklist

All PRs Checklist

• Give your pull request a meaningful name.
• Use lowercase filenames.
• Apply at least one team label according to which team is the content expert (ie. team-FE or team-SAZ)
• Pull request approval from the FE team or content experts (see label applied above) that isn't the content creator.

Content Changes Checklist

• Check that your article looks correct in the preview here or in a Netlify deploy preview.
• Check the links in your article.
• Check the images in your article (if there are any)
• Check to make sure you are using markdown appropriately as outlined in examples/article.md in the root of the project directory and on the momentum doc's preface article
• Check to make sure the Copy and Tone Guidelines are followed.

[netlify]

👷 Deploy request for support-docs pending review.

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