This can be used for bulk tagging of bugs identified by some other
search filters. For example, for proactive backports effort, the script
can be used to mark identified candidates for backports in Launchpad for
later triaging.
While at it, for docs, made it a bit more explicit that
bugs-fixed-since.py script requires more parameters than just --start.
Change-Id: Icadacf534b03336dae6516eab59f7627c996ad80