RE: Info viewer: Links to symbol documentation

[Drew Adams]

> +1 from me.  Perhaps some users will want a toggle.

+1, in general.

1. Needs an option or a minor mode, for on/off.
   IOW, this should not happen in a hard-coded way:

   (add-hook 'Info-selection-hook 'Inform-make-xrefs)

2. At least in Emacs 26.3, I don't see the `Inform-echo'
   text (and I don't see where property `Inform-echo' is
   actually used in the code).  Instead, I see, for each
   link ("button"), this tooltip: "mouse-1, RET: Push
   this button".

3. It could help to (at least optionally be able to)
   have different link faces for the different types of
   objects (options, other variables, commands, other
   functions, etc.).

4. Using `mouse-1' or `RET' a second time could remove
   the *Help* display.  IOW, let it be a toggle for
   showing *Help* about the given object.