Clarify output-path message

On a past meeting where @m4rc1e explained the process of going VF fonts, @bghryct and @thundernixon observed the help message for output-path only mentions UFO as an input file. Despite there is an 'e.g.' introduction, it could lead to confusion cause it makes it sound like the input has to be a UFO.

It could be improved by adding a mention to glyphs, something like:
`Output font file path. Only valid when the output is a single file (e.g. input is a single UFO, Glyphs file, or the output is a variable font)`