doc: more detail on when timers are executed
Checklist
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documentation is changed or added -
the commit message follows commit guidelines
Affected core subsystem(s)
doc
Description of change
This is an attempt to add information about when timers are executed in the event loop. I have no idea if this is correct. I've attempted to incorporate what I found in https://nodesource.com/blog/understanding-the-nodejs-event-loop/ and #7145 (closed).