we're using prpc 7.1.6 in production and we upgraded to 7.1.9 in our dev and QA environment. We're testing QA but we just recently noticed the local online help site is not available even thought the prhelp.war is deployed correctly. It is not available in our (prpc 7.1.6) production environment also. (it doesn't look like anyone is using it)
Anyway, so when I go to http://<hostname>:8080/prhelp I get a 404 "resource not found" error from jboss.
I don't think you did anything wrong. Normally you would be accessing prhelp from within the developer portal where the procomhelpmain.htm will be appended to prhelp automatically. The prhelp web app does not include index.htm or index.jsp that is why direct access will throw 404 error.
I think you have missed to configure the online help URL as specified by Venkatesh Guntupalli. Once the online help url is updated in System --> Resource URLs landing page. The developer portal's help URL would be https://<hostname>:<portnumber>/prhelp/procomhelpmain.htm. But from your screenshot only https://procomhelpmain.htm is there in the address bar.
Please configure the online help URL and share your thoughts.