docs: improve qcow2 spec about extending image header

docs: improve qcow2 spec about extending image header

Make it more obvious how to add new fields to the version 3 header and
how to interpret them.

The specification is adjusted so that for new defined optional fields:

1. Software may support some of these optional fields and ignore the
   others, which means that features may be backported to downstream
   Qemu independently.
2. If we want to add incompatible field (or a field, for which some of
   its values would be incompatible), it must be accompanied by
   incompatible feature bit.

Also the concept of "default is zero" is clarified, as it's strange to
say that the value of the field is assumed to be zero for the software
version which don't know about the field at all and don't know how to
treat it be it zero or not.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200131142219.3264-2-vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
[mreitz: s/some its/some of its/]
Signed-off-by: Max Reitz <mreitz@redhat.com>