Add note about correct calling of wallcycle counter

The code could be refactored to use a series of explicit OpenMP
barriers so that the wallcycle_start/stop are more clearly matched,
but that's too hard for now. The comments make it harder for anyone
to misunderstand or mistakenly change the code.

Change-Id: Iec00e50e26a60efe09afb6f5744d401a9d9c06a3