Fixed #29756 -- Doc'd that model field names can't end with an underscore.

docs/topics/db/models.txt

@@ -649,7 +649,7 @@ just refer to the other model class wherever needed. For example::
 Field name restrictions
 -----------------------
 
-Django places only two restrictions on model field names:
+Django places some restrictions on model field names:
 
 1. A field name cannot be a Python reserved word, because that would result
    in a Python syntax error. For example::
@@ -663,6 +663,8 @@ Django places only two restrictions on model field names:
        class Example(models.Model):
            foo__bar = models.IntegerField() # 'foo__bar' has two underscores!
 
+3. A field name cannot end with an underscore, for similar reasons.
+
 These limitations can be worked around, though, because your field name doesn't
 necessarily have to match your database column name. See the
 :attr:`~Field.db_column` option.