qapi/block-core: Improve MapEntry documentation

MapEntry and BlockDeviceMapEntry are kind of the same thing, and the
latter is not used, so we want to remove it.  However, the documentation
it provides for some fields is better than that of MapEntry, so steal
some of it for the latter.

(And adjust them a bit in the process, because I feel like we can make
them even clearer.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201104165513.72720-2-mreitz@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>