add comment about splitting update expression by operator keywords

moto/dynamodb2/models.py

@@ -116,7 +116,10 @@ class Item(BaseModel):
         }
 
     def update(self, update_expression, expression_attribute_names, expression_attribute_values):
+        # Update subexpressions are identifiable by the operator keyword, so split on that and
+        # get rid of the empty leading string.
         parts = [p for p in re.split(r'\b(SET|REMOVE|ADD|DELETE)\b', update_expression) if p]
+        # make sure that we correctly found only operator/value pairs
         assert len(parts) % 2 == 0, "Mismatched operators and values in update expression: '{}'".format(update_expression)
         for action, valstr in zip(parts[:-1:2], parts[1::2]):
             values = valstr.split(',')