ixgbe: Document how to enable NBASE-T support

Commit a296d665 ("ixgbe: Add ethtool support to enable 2.5 and 5.0
Gbps support") introduced suppression of the advertisement of NBASE-T
speeds by default, according to Todd Fujinaka to accommodate customers
with network switches which could not cope with advertised NBASE-T
speeds, as posted in the E1000-devel mailing list:

https://sourceforge.net/p/e1000/mailman/message/37106269/



However, the suppression was not documented at all, nor was how to
enable NBASE-T support.

Properly document the NBASE-T suppression and how to enable NBASE-T
support.

Fixes: a296d665 ("ixgbe: Add ethtool support to enable 2.5 and 5.0 Gbps support")
Reported-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>