qapi/block-core: Improve x-blockdev-change documentation
The description of feature @unstable is three paragraphs. The second
and third became part of the description by accident in commit
9fb49daabfb (qapi: Mark unstable QMP parts with feature 'unstable').
The second paragraph describes a defect in terms of the
implementation. Fine, but doesn't belong into user-facing
documentation. Turn it into a TODO section.
Rewrite everything else for clarity and completeness.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <
20250311131715.
1296101-1-armbru@redhat.com>
Acked-by: Alberto Garcia <berto@igalia.com>