Update about-powerapps-perapp.md

Adding a clarifying note to step 3 as a follow up to feedback from this issue: https://github.com/MicrosoftDocs/power-platform/issues/513

Author: alaug

power-platform/admin/about-powerapps-perapp.md

@@ -82,6 +82,8 @@ Follow these steps to turn off assigning per app plans for users when an app is
    ![Pass assignment](media/pass-assignment2.png "Pass assignment")
 
 > [!IMPORTANT]
+> If per app plans are not allocated to the environment the app is located, the toggle is disabled. 
+> 
 > Turning off the per app plan is currently available only for canvas apps. Model-driven apps and portals will have this ability in the future.
 
 
@@ -166,4 +168,4 @@ This is an artifact of previous, but no longer required, licenses being assigned
 The maker should sign up for the trial to proceed with creating the connection that uses a gateway. 
 
 
-[!INCLUDE[footer-include](../includes/footer-banner.md)]
+[!INCLUDE[footer-include](../includes/footer-banner.md)]