Add note on overcharge

What does this MR do?

Most custom roles are considered billable users that use a seat. This MR adds documentation stating that when you add a user to your group with a custom role, a warning is displayed if you are about to incur additional charges for having more seats than are included in your subscription.

This is now able to be merged as !136860 (merged) is merged.

Related issues

!136860 (merged)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Style Guide.
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• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
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