[wpilib] Remove Timer lock in wpilibj and update docs (#3602)

Timer reports a negative duration if the sim timing is restarted. This
can be worked around by calling Reset(). Other options included:

1. Have RestartTiming() call Timer::Reset() on a list of instantiated
   Timers.
2. Have Timer::Get() reset the timer if it notices time went backwards.
   This requires dropping const qualification though, which is a
   breaking change that only fixes a minor edge case.

Closes #2732.

wpilibc/src/main/native/include/frc/Timer.h

@@ -29,7 +29,10 @@ void Wait(units::second_t seconds);
 units::second_t GetTime();
 
 /**
- * A wrapper for the frc::Timer class that returns unit-typed values.
+ * A timer class.
+ *
+ * Note that if the user calls frc::sim::RestartTiming(), they should also reset
+ * the timer so Get() won't return a negative duration.
  */
 class Timer {
  public: