Update Event Queue Documentation

[Eric-A-Marks]

Summary of changes

Updates the event queue documentation to clarify that canceling dispatched events is unsafe.

Impact of changes

Migration actions required

Documentation

Related: #14405
Related: #14422
Resolves: #14217

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[X] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@Eric-A-Marks, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[adbridge]

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[mbed-ci]

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