Document caveats with callable references in migrations

[Piotr Maliński]

Model fields support callables in few places, like:

image = models.ImageField(db_index=True, upload_to=upload_path, blank=True)

where upload_path is a function. Migrating model with such field will give:

 ('image', models.ImageField(blank=True, upload_to=my_app.models.upload_path)),

That can easily cause problems. Future changes to the code (like different callable name) would break old migrations (which then would break tests). Does migrations must copy everything?

[Tim Graham]

I believe we'll just want to document the caveats here. docs/topics/migrations/#serializing-values looks like a reasonable place to do so.

[Piotr Maliński]

Well, if so then in general those new migrations will require much more time for maintenance than south migrations. If such callable is changed then either migrations must be edited or squashed and edited. And there is more cases where new migrations make such problems.

[Tim Graham <timograham@…>]

In 50ae4b4b9e146848ce0d95e76662ec0cf667fbe6:

Fixed #23131 -- Documented serialization of callable functions in migrations.

[Tim Graham <timograham@…>]

In 8c8ce2ee64b098dc52ebda7ce95644cb7c2f9cb2:

[1.7.x] Fixed #23131 -- Documented serialization of callable functions in migrations.

Backport of 50ae4b4b9e from master