A comment has be put in the documentation at https://forge.fi-ware.org/plugins/mediawiki/wiki/fiware/index.php/Publish/Subscribe_Broker_-_Orion_Context_Broker_-_User_and_Programmers_Guide#Convenience_Update_Context, but it is mostly a "hook" to be complete as part of the solution of #225.

NTC (in this PR).