We could add a "View in Manual" button in Help buffers if info-lookup
says that the symbol is in the Emacs Lisp Info index.
Yes, it should be fairly easy to do using the
`help-fns-describe-*-functions` hooks.
For interactive functions (a.k.a. "commands"), there's an ambiguity
here: should the link lead to the Emacs user manual or to the Emacs
Lisp Reference manual? We could show both, but that could confuse
users (as opposed to Lisp programmers).
I think something like:
Also documented in the >Emacs user's manual<.
Also documented in the >ELisp programmer's reference<.
should be clear enough (where >...< is meant to delimit the hyperlink).