doc: Clarify the meaning of VFMMics
doc: Clarify the meaning of VFMMics

Also other tweaks to VFM's documentation.

Kamekameha/vapoursynth,vapoursynth/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth