bug#12000: 24.1.50; do not hard-code `bold' font for help output

[Drew Adams]

Two things to add, for clarification:

1. There is an overlay on this text that uses face `button'.  But face `bold' is
also used on the text, and is hard-coded in `describe-mode':

(add-text-properties 0 (length pretty-minor-mode)
                     '(face bold) pretty-minor-mode)

It is the hard-coding of `bold' here that is objectionable.


2. The screenshot I sent was actually from Emacs 23.3.  But the problem has not
changed for Emacs 24 (hard-coded face `bold' remains).

The only difference is that now face `button' inherits from face `link', so the
appearance is a bit different (blue, not black).