qapi/block-core: improve Qcow2OverlapCheckFlags documentation

qapi/block-core: improve Qcow2OverlapCheckFlags documentation

Most of fields have no description at all. Let's fix that. Still, no
reason to place here more detailed descriptions of what these
structures are, as we have public Qcow2 format specification.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20240325120054.2693236-1-vsementsov@yandex-team.ru>
Acked-by: Markus Armbruster <armbru@redhat.com>
[Capitalize "QEMU", update qapi/pragma.json]
Signed-off-by: Markus Armbruster <armbru@redhat.com>