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

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

Author: eyalgal

content/admin/managing-iam/iam-configuration-reference/username-considerations-for-external-authentication.md

@@ -69,9 +69,12 @@ When you provision a new user from your identity provider, the new {% data varia
 
 If you use {% data variables.enterprise.data_residency %}, when you create an {% data variables.enterprise.prodname_emu_enterprise %} on {% data variables.enterprise.data_residency_site %}, your enterprise's shortcode is randomly generated.
 
-* The shortcode is **not** used as a suffix in the usernames of provisioned users.
+* Although the shortcode is not visible in the user interface, it is still appended as a suffix to the usernames of provisioned users. This suffix is not displayed to the users in the UI, but it remains part of their usernames.
 * The only place you are likely to see the shortcode is in the username of the setup admin, which will look like `2abvd19d_admin`.
 
+> [!NOTE]
+> Due to the inclusion of the hidden shortcode, the character limit for usernames is reduced from 39 characters to 30 characters for GitHub Enterprise Cloud with Data Residency.
+
 ## About normalized usernames
 
 Usernames are formed by normalizing the SCIM `userName` attribute value sent from the IdP.