bug#5474: 23.1.91.1; describe-mode does not offer link to the .el

[jidanni]

We hit C-h m. We see the following:

  XXXX mode:
  Major mode for editing XXXX.

  key             binding
  ---             -------

  C-c           Prefix Command
  ESC           Prefix Command

  C-M-x         XXXX-eval-defun

Nowhere is there a link to XXXX.el.
We have to hit XXXX-eval-defun to get a link to XXXX.el.

It should say at top
  XXXX mode:
  Major mode for editing XXXX. Defined in XXXX.el <---clickable.

Or
  XXXX mode (... XXXX-mode, defined in XXXX.el):
  Major mode for editing XXXX.