Version:            (using KDE 4.2.0)
OS:                Linux
Installed from:    Ubuntu Packages

The only user-accessible documentation for the Applications plugin seems to be the one-line description in the krunner Settings window, which merely says:
  Find applications, control panels and services

The user is left without any hint about the following:
  1. where does it look (and how can one change this)?
  2. What does it define as an "application, control panel or service"? [I honestly have no idea what it means by "control panel" or "service"; those needs to be clearly described in the plugin documentation too; they are not terms that I, as a long-time *nix user, recognise.]
 
Experimenting doesn't help. If I type in "acr", for example, (being the first three characters of "acroread, which is installed on my system), I get no results. Similarly if I type "soffice", I get no hits, even though typing it in a terminal brings up OpenOffice.org. 

So without documentation essentially the user is flying blind, with no idea as to whether the plugin is going to find a particular application.