bug#18992: 24.4; describe-mode

[Drew Adams]

> What about mentioning the variable at stake, i.e. "major-mode: "
> resp. "minor-mode: " in format-function?
> As said, repeating the rather arbitrary mode-line string doesn't
> contribute, is confusing.

There is no variable `minor-mode'.  And users can find out about 
`major-mode' elsewhere.