describe-mode should mention the .el file of the mode

[Dan Jacobson]

We hit C-h m, describe-mode, on your average mode:
The major mode is described first.
For minor modes, see following pages.

[average] mode:
bla bla bla

Well, aren't you going to tell us what .el file this mode is defined
in, and make it a clickable link so one can go straight to see the
file?