Revision 222678

MFC r222447:

Mention that jumbo frame support is disabled on PCIe VT6130/VT6132
controllers because of TX MAC hangs when trying to send a frame
that is larger than 4K (see r200759).

PR:		docs/156742
Submitted by:	Michael Moll (kvedulv at kvedulv dot de)
Reviewed by:	yongari@