add custom root roles to docs on private projects (#5829)

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Co-authored-by: Thomas Heartman <[email protected]>

website/docs/reference/project-collaboration-mode.md

@@ -27,7 +27,7 @@ Only admins and project members can submit change requests in a **protected** pr
 
 Private collaboration mode renders the project invisible to all **viewers** who are not project members. This means that **viewers** cannot see the project in the project list, nor can they access the project's features or locate the project anywhere within Unleash.
 
-**Editors** and **admins** can still see private projects.
+**Editors**, **admins**, and users with **custom root roles** can still see private projects.
 
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