I think you might have some problems using the f1 key. Both IE and FireFox, for instance, intercept it to open their own Help pages. But you could easily place a Dokuwiki-style button at both the top and bottom of the page and/or a link/button in a sidebar which does the job.
Every Dokuwiki page displayed in the browser or opened for editing is accessible through the global $ID. Your button can be made aware of the current page $ID and open the appropriate help pages in the browser window by accessing the help page using the help page's id in a URL, just as Dokuwiki does. It could even pop up a list of relevant help pages as links.
For the Dokuwiki environment variables see
http://www.dokuwiki.org/devel:environment#global_variables