add example of preference key construction

Author: mdavidsaver

docs/source/conf.py

@@ -333,6 +333,11 @@ def createPreferenceAppendix(root):
 The following preference settings are available for the various application features.
 To use them in your settings file, remember to prefix each setting with the package name.
 
+eg. for the 'drop_failed_archives' preference in the 'trends.databrowser3' section
+(package name 'org.csstudio.trends.databrowser3') add a line like: ::
+
+    org.csstudio.trends.databrowser3/drop_failed_archives=true
+
 """)
         for pack in sorted(pref_files.keys()):
             pref_file = pref_files[pack]