qapi/block-core: Improve x-blockdev-change documentation

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>