Tidy up filmic documentation

[elstoc]

Filmic documentation has become rather sprawling and hard to read (especially given the large number of iterations -- up to v7 now). We should certainly at least consider splitting up the rather absurd "mid-tones saturation / extreme luminance saturation / highlights saturation mix" control into separate definitions (they are all the same parameter renamed for different uses depending on version).

Also, even after several copy-edits there is still a bunch of Franglais that needs fixing.