docs: clarify timer accumulation in $effect tutorial

[mdmahendri]

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What does this PR do?

• Clarifies the explanation about the interval timer example in the tutorial
• Explain in more detail that slow down doesn't work because multiple timers accumulate

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[Rich-Harris]

Thank you but this feels unnecessary — the text already explains what's happening. In fact the proposed change is less clear: whereas at present it says that the effect runs 'each time interval gets smaller', the changed text connects the effect to the event (the button click) rather than the state change that happens as a result of the event.