Document plTimerShare

[Hoikas]

When working on #1453, I had to dig into the code a bit to understand what the difference between hsTimer::GetSeconds() and hsTimer::GetSysSeconds() is. It would be nice to have these methods documented in the header file. Tangentially, considering that the code was arguably using the wrong kind of seconds previously, it may be worth auditing calls to hsTimer to ensure the correct clock is being used. This is currently not a big deal because the time scale factor because the time scale factor is always 1.0.