Migrations won't work if field signature changes

[Markus Holtermann]

If the __init__ signature of a model field changes in the way that e.g. a positional argument is added / removed and doesn't replace / isn't replaced by a previously existing keyword argument, the old migration files won't work any more because the rely on the actual __init__ signature to create an instance of the field.

This should be documented in the respective custom field and migration docs.

[rjasjukaitis]

I added a paragraph to the migration documentation. Asking for review: ​https://github.com/django/django/pull/1985

[Raphael Jasjukaitis <r.jasjukaitis@…>]

In 455e2896b122a331057483634bea9c8074bdc97d:

Fixed #21499 -- Added a paragraph to the docs.

[Andrew Godwin <andrew@…>]

In 0c46ca83e820cd725f5d0fa299c2b5405bfeee1b:

Merge pull request #1985 from raphaa/21499

Fixed #21499 -- Migrations won't work if field signature changes