docs: fix descriptions in discoverable partitions

docs: fix descriptions in discoverable partitions

00db9a114e ("docs: generate table from header using a script") got the
descriptions for the partition types mixed up.  After that change, the
spec claimed, for example, that the /usr partition should contain
"dm-verity integrity hash data for the matching root partition", and
that the /usr verity partition should be of type "Any native, optionally
in LUKS".  This made the spec an extremely confusing read before I
figured out what must have happened!

I've gone through the table as it existed prior to 00db9a114e, and moved
the descriptions around in the script that generates the table until
they matched up with what they used to be.  Then I regenerated the
table from the fixed script.