Add note for higher RPM variance tolerance for redux line fans

[emansom]

I've moved my X11SSL-F board to another case and equipped it with lower quality NF-P12 redux-1700 PWM (previously used NF-A12x25). On some boots the IPMI complained about too low and too high RPM errors.

I had configured the fans to their correct RPM values (with additional +10% on the upper and -10% on the lower, to account for the tolerance defined by Noctua specs themselves) using a locally modified set_ipmi_threshold.sh (modified for specific fan specs targeted at specific FAN headers, visually inspected).

I have now altered the IPMI fan upper and lower values to account for a higher 25% variance tolerance and thus far no error has appeared in the SEL. Leading me to believe the redux line of Noctua fans need a higher variance tolerance.

[emansom]

An additional warning note in the configuration file to warn users not to decrease the min level below 10% (25% for redux line) and the max level not above 90% (75% for redux line) would also be needed to make sure the fans don't accidentally run out of spec.