Update cross-platform invites documentation for sessions

Document that the session name must currently be `GameSession` for cross-platform invites to work. This is a temporary requirement until sessions has been refactored to use the rooms system.

Author: hach-que

docs/setup/crossplatform/invites.mdx

@@ -20,6 +20,7 @@ To have cross-platform invites work with sessions, you'll need to:
 - Ensure that "Presence Advertises" is to set "Session" in Project Settings.
 - Ensure that "Should Advertise" is enabled when the session is created.
 - Ensure that "Uses Presence" is enabled when the session is created.
+- The session name must be `GameSession`.
 
 Once this is done, the plugin will automatically advertise it on the local platform.