docs/atomic-rollbacks: Add a section on rollbacks

[ericcurtin]

Describing how different types of rollbacks work.

[ericcurtin]

Here is how is looks in .pdf format:

Atomic Rollbacks _ ostreedev_ostree.pdf

[cgwalters]

This looks fine, that said...now that we've done all this I am still coming around to the idea that ostree should automatically inject something like ostree.fallback=/path/to/root for all non-default entries, that way we can reliably detect this from the initramfs and properly set up the default target from the initramfs.

Anyways, something to just consider for later.

[ericcurtin]

We normally fallback at the bootloader level right? So if we have loaded a kernel+initrd+cmdline from a fallback boot entry the ostree= karg would have the fallback root and ostree.fallback= would have the next fallback if that makes sense.

Also... cmdlines are generated server-side, this wouldn't be able to account for clients that missed an upgrade.

You can skip an upgrade in ostree right, if you haven't upgraded in a while?

That's why it's kinda nice to have the /boot/loader data client-side, fallbacks seem like something that should be client-side determined. Because each client may have a different upgrade path (and different fallback versions, or the fallback could be on A on some clients and B on others).

[ericcurtin]

Addressed comments and re-pushed