docs: mention Jitting phase in How it works

[fuguiKz]

Fixes #2635.\n\nAdds a brief note about the Jitting iteration stage (pre-JIT/tiered JIT warmup) to the "How it works" guide.

[fuguiKz]

Good point — I updated the pseudocode to call JittingStage(method) and added a JittingStage method stub for consistency.

[timcassell]

Suggested change

	RunIteration(method, invokeCount: 1, unrollFactor: 1);
	RunIteration(method, invokeCount: 1, unrollFactor: 1);
	if (JitInfo.IsTiered)
	{
	for (int i = 0; i < JitInfo.MaxTierPromotions; i++)
	{
	RunIteration(method, invokeCount: JitInfo.TieredCallCountThreshold, unrollFactor: 1);
	Thread.Sleep(250);
	}
	}