kcms/fonts: make Force Fonts DPI tooltip text more accurate and clearer

This UI control currently has an extremely long tooltip that talks about
a bunch of technical jargon the user is likely to know nothing about,
and also says inaccurate things by mentioning hardware where the DPI is
not automatically detected, when in fact Qt software explicitly avoids
scaling things by DPI in the first place.

BUG: 445647
FIXED-IN: 5.24