Explicitly differentiate between data export and data migration

[benpate]

Data export is the first step in data migration. But there are many valid reasons to just stop there and not "finalize" the migration with Move activities -- such as data backup, or contingency planning. I think the spec should mention this explicitly, giving users the option to leave the data exported without completely migrating their account.

If there's consensus on this, I'm happy to write a PR for this issue.

[lisad]

Yes I absolutely agree with this philosophy but on looking at the doc it's not clear at all in the doc so far. In the Approach Overview I provided the last two steps as fully optional, but didn't explain much why I guess.

I did cover backup with the warning that account portability may not be always appropriate for backup. If the user is doing account portability between two instances of the same implementation, it might work great for backup, but if the two instances are the same implementation then an interoperability standard is less important in the first place. On the third hand, it should be possible to write a good quality specialized backup tool that uses this specification to interoperate with a number of ActivityPub servers.

Anyway that's just a couple notes and I would love to get this explanation improved.