CoreTiming: Document initial startup behavior

Events don't update the downcount until after the first Advance(),
thus Advance() must be called once before scheduling works normally.

Source/Core/Core/CoreTiming.cpp

@@ -129,6 +129,11 @@ void Init()
   g_slice_length = MAX_SLICE_LENGTH;
   g_global_timer = 0;
   s_idled_cycles = 0;
+
+  // The time between CoreTiming being intialized and the first call to Advance() is considered
+  // the slice boundary between slice -1 and slice 0. Dispatcher loops must call Advance() before
+  // executing the first PPC cycle of each slice to prepare the slice length and downcount for
+  // that slice.
   s_is_global_timer_sane = true;
 
   s_ev_lost = RegisterEvent("_lost_event", &EmptyTimedCallback);

Source/Core/Core/CoreTiming.h

@@ -31,6 +31,8 @@ extern u64 g_fake_TB_start_ticks;
 extern int g_slice_length;
 extern float g_last_OC_factor_inverted;
 
+// CoreTiming begins at the boundary of timing slice -1. An initial call to Advance() is
+// required to end slice -1 and start slice 0 before the first cycle of code is executed.
 void Init();
 void Shutdown();
 
@@ -60,12 +62,22 @@ enum class FromThread
 };
 
 // userdata MAY NOT CONTAIN POINTERS. userdata might get written and reloaded from savestates.
+// After the first Advance, the slice lengths and the downcount will be reduced whenever an event
+// is scheduled earlier than the current values (when scheduled from the CPU Thread only).
+// Scheduling from a callback will not update the downcount until the Advance() completes.
 void ScheduleEvent(s64 cycles_into_future, EventType* event_type, u64 userdata = 0,
                    FromThread from = FromThread::CPU);
 
 // We only permit one event of each type in the queue at a time.
 void RemoveEvent(EventType* event_type);
 void RemoveAllEvents(EventType* event_type);
+
+// Advance must be called at the beginning of dispatcher loops, not the end. Advance() ends
+// the previous timing slice and begins the next one, you must Advance from the previous
+// slice to the current one before executing any cycles. CoreTiming starts in slice -1 so an
+// Advance() is required to initialize the slice length before the first cycle of emulated
+// instructions is executed.
+// NOTE: Advance updates the PowerPC downcount and performs a PPC external exception check.
 void Advance();
 void MoveEvents();