diff --git a/scrapy/trunk/scrapy/core/exceptions.py b/scrapy/trunk/scrapy/core/exceptions.py
index bf4fd7fd5..47decc140 100644
--- a/scrapy/trunk/scrapy/core/exceptions.py
+++ b/scrapy/trunk/scrapy/core/exceptions.py
@@ -1,5 +1,8 @@
 """
 Scrapy core exceptions
+
+These exceptions are documented in docs/ref/exceptions.rst. Please don't add
+new exceptions here without documenting them there.
 """
 
 # Internal
@@ -44,4 +47,4 @@ class DropItem(Exception):
 
 class NotSupported(Exception):
     """Indicates a feature or method is not supported"""
-    pass
\ No newline at end of file
+    pass
diff --git a/scrapy/trunk/scrapy/core/signals.py b/scrapy/trunk/scrapy/core/signals.py
index c946856e7..b8178853d 100644
--- a/scrapy/trunk/scrapy/core/signals.py
+++ b/scrapy/trunk/scrapy/core/signals.py
@@ -1,6 +1,10 @@
 """
 Scrapy core signals
+
+These signals are documented in docs/ref/signals.rst. Please don't add new
+signals here without documenting them there.
 """
+
 from pydispatch import dispatcher
 
 from scrapy import log