signals: Document the weird unused import for signal registration.

zerver/apps.py

@@ -16,6 +16,11 @@ class ZerverConfig(AppConfig):
     name = "zerver"  # type: str
 
     def ready(self) -> None:
+        # We import zerver.signals here for the side effect of
+        # registering the user_logged_in signal receiver.  This import
+        # needs to be here (rather than e.g. at top-of-file) to avoid
+        # running that code too early in Django's setup process, but
+        # in any case, this is an intentionally unused import.
         import zerver.signals
 
         if settings.POST_MIGRATION_CACHE_FLUSHING: