spapr: Explain purpose of ->fwnmi_migration_blocker more clearly

spapr: Explain purpose of ->fwnmi_migration_blocker more clearly

spapr_mce_req_event() makes an effort to prevent migration from
degrading the reporting of FWNMIs.  It adds a migration blocker when
it receives one, and deletes it when it's done handling it.  This is a
best effort.

Commit 2500fb423a "migration: Include migration support for machine
check handling" tried to explain this in a comment.  Rewrite the
comment for clarity, and reposition it to make it clear it applies to
all failure modes, not just "migration already in progress".

Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Aravinda Prasad <arawinda.p@gmail.com>
Cc: Ganesh Goudar <ganeshgr@linux.ibm.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-4-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>