Add note regarding firmware update for MGE XML devices with values that are 10x too high

[clepple]

See networkupstools/nut#394 - if I am not mistaken, the DDL has captures from devices with old firmware (but the captures were taken with old versions of NUT that included this scale factor).

Affected DDL entries:

• https://github.com/networkupstools/nut-ddl/blob/master/Eaton/Eaton__Eaton_5PX__netxml-ups__2.7.4__01.dev
• https://github.com/networkupstools/nut-ddl/blob/master/Eaton/Eaton__Eaton_9PX__netxml-ups__2.7.4__01.dev

[jimklimov]

No strong opinion on these... may have to experiment with similar devices. To say the least, the numbers in those files seem to add up (e.g. order of magnitude for voltage*current vs power). But also they lack a decimal point :-\