Help cross reference to source file

["Drew Adams" <drew.adams@oracle.com>]

I have a macro that I use to define functions; that is, it expands to and
evaluates a defun. Suppose that I use the macro in file `foo.el' to define
function `fun'. If I then do C-h f fun, I see this in *Help*:

 fun is an interactive compiled Lisp function in `foo.el'.

The library name foo.el is a link here. However, if I click the link, I get
an error message:

 Cannot find definition of `fun' in library `foo.el'

Fair enough; help can't know where the function definition is, though it is
able to identify the file.

Even if help can't take me to the right place in the source file, I think it
should open the file - and provide the error message, inviting me to search
for it in the file.

This would be analogous to what we do when Emacs cannot find the source C
file that defines a built-in - at least on Windows, it opens a file dialog
box that lets you navigate to the source file.

I think it's great that the source file is identified (correctly), and it's
good that the file name is a link. However, if the definition cannot be
located in the file, the file should be opened, and then a message can say
that the definition can't be located.