Discourage modifying UserNamespace class

Because `:namespace` factory implicitly meant
'User' namespace, we want to ensure existing tests
are still testing against the intended functionality.

app/models/namespaces/user_namespace.rb

@@ -3,6 +3,26 @@
 # TODO: currently not created/mapped in the database, will be done in another issue
 #       https://gitlab.com/gitlab-org/gitlab/-/issues/341070
 module Namespaces
+  ####################################################################
+  # PLEASE DO NOT OVERRIDE METHODS IN THIS CLASS!
+  #
+  # This class is a placeholder for STI. But we also want to ensure
+  # tests using `:namespace` factory are still testing the same functionality.
+  #
+  # Many legacy tests use `:namespace` which has a slight semantic
+  # mismatch as it always has been a User (personal) namespace.
+  #
+  # If you need to make a change here, please ping the
+  # Manage/Workspaces group so we can ensure that the
+  # changes do not break existing functionality.
+  #
+  # As Namespaces evolve we may be able to relax this restriction
+  # but for now, please check in with us <3
+  #
+  # For details, see the discussion in
+  # https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74152
+  ####################################################################
+
   class UserNamespace < Namespace
     def self.sti_name
       'User'