Open docs.racket-lang.org if no offline manual available.
C-c C-d TAB works on my Debian, if i have package racket-doc
installed. It opens URL file:///usr/share/doc/racket/search/index.html?q=hc-append. But can it fall back to online Racket documentation, http://docs.racket-lang.org/search/index.html?q=hc-append, when i don't have offline docs installed?
I'm not sure that Geiser has this feature now, i rgrep
'ed geiser
directory for "racket-lang", it surely isn't hardcoded.