Clarify admin requirement for CPM

[JoeMitchellJones]

Clarifies that users must be org admins to make changes to manage config policies

Description

Adds a line clarifying that users must be org admins to manage config policies

Reasons

Previously, the only way users would know this is by trying and failing, or reading one of our blog posts.
This will reduce confusion about who can manage policies

Content Checklist

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[rosieyohannan]

Thank you, @JoeMitchellJones! I think I'll combine the two notes together so we don't have two banners at the top of the page