Update safe-attachments-policies-configure.md

Adding a note that leaving user, group and domain field blank applies the policy to all users. The documentation was missing clarity on how to create policy that could apply to all users in the org. A customer with large user and hunderds of domain reported this scenario and was a blocker due to missing clarity here.

Author: qaehtesh

defender-office-365/safe-attachments-policies-configure.md

@@ -95,6 +95,7 @@ You configure Safe Attachments policies in the Microsoft Defender portal or in E
 
      > [!TIP]
      > Subdomains are automatically included unless you specifically exclude them. For example, a policy that includes contoso.com also includes marketing.contoso.com unless you exclude marketing.contoso.com.
+     > Also leaving user, group and domain field blank will create a policy that applies to all users.
 
    Click in the appropriate box, start typing a value, and select the value that you want from the results. Repeat this process as many times as necessary. To remove an existing value, select :::image type="icon" source="media/m365-cc-sc-remove-selection-icon.png"::: next to the value.