Fixed #25917 -- Clarified reversibility of RemoveField.

Removes a field from a model.

Bear in mind that when reversed this is actually adding a field to a model;

if the field is not nullable this may make this operation irreversible (apart

from any data loss, which of course is irreversible).

Bear in mind that when reversed, this is actually adding a field to a model.

The operation is reversible (apart from any data loss, which of course is

irreversible) if the field is nullable or if it has a default value that can be

used to populate the recreated column. If the field is not nullable and does

not have a default value, the operation is irreversible.

AlterField

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