pci: Add comment for pci_add_capability2()

Comments for pci_add_capability2() to explain the return
value. This may help to make a correct return value check
for its callers.

Cc: mst@redhat.com
Cc: marcel@redhat.com
Cc: armbru@redhat.com
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
