docs/devel: Do not unparent in instance_finalize()

docs/devel: Do not unparent in instance_finalize()

Children are automatically unparented so manually unparenting is
unnecessary.

Worse, automatic unparenting happens before the instance_finalize()
callback of the parent gets called, so object_unparent() calls in
the callback will refer to objects that are already unparented, which
is semantically incorrect.

Remove the instruction to call object_unparent(), and the exception
of the "do not call object_unparent()" rule for instance_finalize().

Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Link: https://lore.kernel.org/r/20250924-use-v4-1-07c6c598f53d@rsg.ci.i.u-tokyo.ac.jp
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>