docs: clarify member-only profile instructions

The docs do specify a private repository already, but this adds a callout to emphasize that this doesn't mean "not public" but rather that it has to have visibility of private -- internal is not enough.

Author: mbainter

content/organizations/collaborating-with-groups-in-organizations/customizing-your-organizations-profile.md

@@ -56,6 +56,8 @@ The content of public `README.md` will appear on your organization's public prof
 
 ## Adding a member-only organization profile README
 
+>[!NOTE] Private organization profiles must have "Private" visibility, not "Internal".
+
 The content of a member-only `README.md` will be displayed in the member view of your organization's profile.
 
 1. If your organization does not already have a `.github-private` repository, create a private repository called `.github-private`.