docs: clarify member-only profile instructions

[mbainter]

Why:

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. It can be easy to think of internal as a different level of visibility for a private repository instead of a whole other type that affects the way other features interact with it.

Because of how we structure our organization I wanted our .github-private repo to be internal and for a while I could not figure out why the org profile wasn't showing. Since we had also moved to Enterprise Managed Users I defaulted to assuming it was connected to EMU and glossed over the visibility as a factor. Trying to do searches when troubleshooting this problem also didn't yield helpful results, partly because I assumed it was connected to EMUs. I almost moved on once I figured it out, but I suspect maybe I'm not the first or the last to be bitten by this. I figure worst case people searching on it might land here.

I also considered rewording the documentation itself, but I think this is more a problem of ensuring people have the right mental modal for private vs internal. I don't know if that has the same impact anywhere else, so a small callout to help people avoid making my mistake seemed like the best solution to me.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adding a callout to the member-only profile instructions

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
organizations/collaborating-with-groups-in-organizations/customizing-your-organizations-profile.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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