Timers cannot be reliably cancelled or nested prior to 6.6.0

We need to document and release-note the below limitation in 5.5.x, 6.0.x and 6.5.x codelines:

Eventing timers once created cannot be overwritten by using the same reference. In addition, a function that is invoked by a timer callback cannot create a fresh timer. Both limitations are addressed in 6.6.0 by MB-28734.

In 6.6.0, we need to advertise the following in both release notes and documentation:

Eventing timers can now be cancelled using cancelTimer() function, or by creating a new timer with same reference as an existing timer. In addition, a function that is invoked by a timer callback can create fresh timers.