Module reference docs: automatic migrations: specify that altering an enum type used in a table requires a manual migration

[gefjon]

Reported on Discord, with the suggestion of adding:

Altering (adding, renaming, deleting, reordering) variants of an enum

to the list of things that require a manual migration. I would suggest generalizing this to include redefining any user-defined type, and clarify that it only affects types used as columns of tables (or types used within types used as columns within tables, recursively).