Missing information about model field name restriction

[zdpx]

Documentation available at ​https://docs.djangoproject.com/en/2.1/topics/db/models/#field-name-restrictions says, that there are only 2 restrictions on naming model fields:

1. A field name cannot be a Python reserved word
2. A field name cannot contain more than one underscore in a row

I've found another restriction: field name can't ends with underscore, because it generates error during making migrations:

 (fields.E001) Field names must not end with an underscore.

[Tim Graham <timograham@…>]

In d483a5f:

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