Add vnic_type documentation

Add documentation for added attribute 'binding:vnic_type' for port binding extension.
Possible values are 'direct', 'macvtap' and 'normal'.
'binding:vnic_type' is allowed to be defined on port creation or changed
on port update by admin or tenant user.

Change-Id: I7ba6cbf50453d19d627c5f2a1ca95a9da9e11eba
Closes-Bug: #1283334