Update base permissions docs to explicitly state changes impact new & existing customers.

[adamrr724]

Why:

Closes #24144

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

Adding a note that states base permissions changes take effect on both new and existing customers.

Check off the following:

• I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• For content changes, I have completed the self-review checklist.

[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
organizations/managing-user-access-to-your-organizations-repositories/setting-base-permissions-for-an-organization.md	fpt ghec ghes@ 3.8 3.7 3.6 3.5 3.4 ghae	fpt ghec ghes@ 3.8 3.7 3.6 3.5 3.4 ghae

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

👋 @adamrr724 Thanks for opening an issue and submitting a PR! I edited your PR slightly, removing the word ISSUE after the Closes prompt to link the PR to the issue. 👍

I'll get this triaged for review! ⚡

[sophietheking]

@adamrr724 👋 thanks for opening this PR! I left a suggestion to update the note slightly.

Please feel free to @mention me here to get this merged!

[adamrr724]

Hi @sophietheking! All committed and ready for merge. Thank you so much!

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