Add comments to fontval check downgrades and silencing and
Add comments to fontval check downgrades and silencing and

also fix the implementation of silencing them.
(issue #2102)

googlefonts/fontbakery,graphicore/fontbakery,googlefonts/fontbakery,moyogo/fontbakery,moyogo/fontbakery,googlefonts/fontbakery,graphicore/fontbakery,moyogo/fontbakery,graphicore/fontbakery