docs: clarify bookerReopened and bookerReloaded are for prerendered modals

[hariombalhara]

Summary

Updates the embed events documentation to clarify that bookerReopened and bookerReloaded events are only applicable for prerendered modals that are now visible.

This adds a single clarifying sentence to each event's description to help developers understand when these events will fire.

Review & Testing Checklist for Human

• Verify the technical accuracy: confirm that bookerReopened and bookerReloaded events truly only fire for prerendered modals that are now visible (not for non-prerendered modals)
• Review the wording to ensure it's clear to developers integrating with the embed system

Notes

• Link to Devin run: https://app.devin.ai/sessions/35a0c5d4d5fc4feb9a5af2d8d340b9c5
• Requested by: @hariombalhara

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