create_user: Improve comment about prereg_user handling.

The dangling comment was not very helpful and unclear about the pieces
of code it was referring to.

We expand the part about linking the prereg_user to the created user,
while the part about "revoking other preregistration users" is
redundant, because the relevant code block lower down already has
comments on it with better explanations.

zerver/actions/create_user.py

@@ -235,9 +235,9 @@ def process_new_human_user(
                 ),
             )
 
-    # Revoke all preregistration users except prereg_user, and link prereg_user to
-    # the created user
-
+    # For the sake of tracking the history of UserProfiles,
+    # we want to tie the newly created user to the PreregistrationUser
+    # it was created from.
     if prereg_user is not None:
         prereg_user.status = confirmation_settings.STATUS_ACTIVE
         prereg_user.created_user = user_profile