Added developer documentation of the normal path locations

Framework/Kernel/src/ConfigService.cpp

@@ -1628,6 +1628,11 @@ const std::string ConfigServiceImpl::getVTPFileDirectory() {
 }
 /**
  * Fills the internal cache of instrument definition directories
+ *
+ * This will normally contain from Index 0 
+ * - The download directory (win %appdata%\mantidproject\mantid\instrument) (linux $home/.mantid/instrument )
+ * - The user instrument area /etc/mantid/instrument (not on windows)
+ * - The install directory/instrument
  */
 void ConfigServiceImpl::cacheInstrumentPaths() {
   m_InstrumentDirs.clear();