Update v2-oauth2-auth-code-flow.md

adding note about default behavior in absence of optional parameters 
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cc: @nickludwig

Author: nickludwig

articles/active-directory/develop/v2-oauth2-auth-code-flow.md

@@ -54,6 +54,8 @@ The authorization code flow begins with the client directing the user to the `/a
 
 Some permissions are admin-restricted, for example, writing data to an organization's directory by using `Directory.ReadWrite.All`. If your application requests access to one of these permissions from an organizational user, the user receives an error message that says they're not authorized to consent to your app's permissions. To request access to admin-restricted scopes, you should request them directly from a Global Administrator. For more information, see [Admin-restricted permissions](v2-permissions-and-consent.md#admin-restricted-permissions).
 
+Unless specified otherwise, there are no default values for optional parameters. There is, however, default behavior for a request omitting optional parameters. The default behavior is to either sign in the sole current user, show the account picker if there are multiple users, or show the login page if there are no users signed in. 
+
 ```http
 // Line breaks for legibility only