Name of the system-assigned identity

Added a section that explains what's the name of the system-created managed identity. This information was missing in various documentation pages and I only found it hidden somewhere in one of the tutorial pages. In my opinion tutorials should not be the only place where important facts are documented.

Author: latusekpiotr

articles/active-directory/managed-identities-azure-resources/overview.md

@@ -46,6 +46,7 @@ There are two types of managed identities:
     - A service principal of a special type is created in Azure AD for the identity. The service principal is tied to the lifecycle of that Azure resource. When the Azure resource is deleted, Azure automatically deletes the service principal for you. 
     - By design, only that Azure resource can use this identity to request tokens from Azure AD.
     - You authorize the managed identity to have access to one or more services.
+    - The name of the system-assigned service principal is always the same as the name of the Azure resource it is created for. For a deployment slot, the name of its system-assigned identity is <app-name>/slots/<slot-name>.
 
 - **User-assigned**. You may also create a managed identity as a standalone Azure resource. You can [create a user-assigned managed identity](how-to-manage-ua-identity-portal.md) and assign it to one or more Azure Resources. When you enable a user-assigned managed identity:      
     - A service principal of a special type is created in Azure AD for the identity. The service principal is managed separately from the resources that use it.