ixgbe: fix documentation

The MTU values in the documentation do not match the source.
The source has frame limit of IXGBE_MAX_JUMBO_FRAME_SIZE (9728)
which is MTU of 9710 because of the accounting for Ethernet header
and CRC.

Also, don't refer to the obsolete ifconfig command.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>