configdb - added commandref docu 

for interaction between configDB and other them modules.
Currently 02_RSS.pm and 95_holiday.pm will use configDB to find their data inside the database.

git-svn-id: svn://svn.code.sf.net/p/fhem/code@5584 2b470e98-0d58-463d-a4d8-8e2adae1ed80