Document Firefox pause/resume edge case for scheduler work state

[Saniya8567-GitHub]

This change documents a Firefox-specific edge case where JavaScript execution
may pause due to debugger breakpoints or alert() calls while scheduler work
is in progress.

When execution resumes, the internal isPerformingWork flag may remain set if
cleanup logic does not run as expected. This comment is related to issue #17355
and is intended to improve maintainability and debugging.

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