For KCM modules, clicking the "Help" button when khelpcenter isn't
installed should open the corresponding documentaion for that module
at docs.kde.org; for example with the Fonts KCM, the "&application="
part of the url should be "&application=kcontorl/fonts".
See D27518 for more details.
Thanks to ltoscano for fixing docs.kde.org to make "&application=kcontrol/<module name>"
actually resolve to the correct url path.