Comment 6 for bug 1558206

I renamed the waitRelease flag to threadDestroyed and rewrote the comments in the epicsThread.cpp file to better explain what's going on. Also added Release Notes linking to this bug.

When committing bug fixes to Bazaar, please use the command-line flag --fixes=lp:<number> which will link that branch to the bug automatically. Just putting 'fixes lp:<number>' into the commit log is not sufficient for Launchpad to notice (but include that too as it's easier to search commit logs).