The command you're looking for is
Info-goto-emacs-command-node, bound to
C-h F. Unfortunately there doesn't seem to be any integration between the two different forms of documentation.
This is a bit rough, but drop this in your
.emacs file and from the
C-h f documentation for a function you'll be able to hit
i ("info") to jump to the corresponding
C-h F documentation.
(defun Info-goto-from-command-help ()
"Go to the Info node in the Emacs manual for the command
currently being viewed in `help-mode'."
(when (eq 'describe-function (car help-xref-stack-item))
(Info-goto-emacs-command-node (cadr help-xref-stack-item))))
(define-key help-mode-map "i" 'Info-goto-from-command-help)