Add description of issue

pandas/_libs/tslibs/timestamps.pyx

@@ -702,7 +702,7 @@ class Timestamp(_Timestamp):
 
     @tz.setter
     def tz(self, value):
-        # GH 3746
+        # GH 3746: Prevent localizing or converting the index by setting tz
         raise AttributeError("Cannot directly set timezone. Use tz_localize() "
                              "or tz_convert() as appropriate")
 

pandas/core/indexes/datetimes.py

@@ -685,7 +685,7 @@ def tz(self):
 
     @tz.setter
     def tz(self, value):
-        # GH 3746
+        # GH 3746: Prevent localizing or converting the index by setting tz
         raise AttributeError("Cannot directly set timezone. Use tz_localize() "
                              "or tz_convert() as appropriate")