Update username-considerations-for-external-authentication.md

[eyalgal]

Improved the explanation of the GHE.com shortcode/username relationship

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• 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.
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[Copilot]

Pull Request Overview

This PR improves the explanation of the relationship between shortcodes and usernames for external authentication.

• Clarifies that while the shortcode is not visible in the UI, it is still appended to the usernames.
• Adds a note that the inclusion of the hidden shortcode reduces the username character limit for GitHub Enterprise Cloud with Data Residency.

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on the review server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
admin/managing-iam/iam-configuration-reference/username-considerations-for-external-authentication.md	ghec ghes@ 3.16 3.15 3.14 3.13 3.12	ghec ghes@ 3.16 3.15 3.14 3.13 3.12

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

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[jc-clark]

Thanks for making this update @eyalgal! ✨

I had a couple minor suggestions below. Let me know what you think and I can help merge this to main.

[eyalgal]

@jc-clark Thank you!

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