Clarifying AAD email address attribute prereq

Adding another sentence for the AAD email address attribute description, just in case admins forget to set up mailboxes that match the attribute they select here (failure to note this caused some internal test cases to be incomplete and fail). Adding this to all platforms.

Author: tycastMSFT

memdocs/intune/configuration/email-settings-android-enterprise.md

@@ -48,7 +48,7 @@ As an Intune administrator, you can create and assign email settings to Android
 
 - **Email app**: Select **Gmail** or **Nine Work**. This app is the client app that connects to the email server you enter.
 - **Email server**: Enter the host name of your Exchange server. For example, enter `outlook.office365.com`.
-- **Username attribute from AAD**: This name is the attribute Intune gets from Azure Active Directory (Azure AD). Intune dynamically generates the username that's used by this profile. Your options:
+- **Username attribute from AAD**: This name is the attribute Intune gets from Azure Active Directory (Azure AD). Intune dynamically generates the username that's used by this profile. Make sure your users have email addresses that match the attribute you select here. Your options:
 
   - **User Principal Name**: Gets the name, such as `user1` or `[email protected]`.
   - **User name**: Gets only the name, such as `user1`.