Improve trace_event documentation

[AndreasMadsen]

• Version: master
• Platform: all
• Subsystem: trace_events, docs

The trace_events documentation is rather short and titled wrong.

We should:

• change title from tracing to trace_event
• describe what each category contains:
  • v8 events are GC, compiling, and execution related
  • node is a placeholder. Doesn't actually contain anything
  • node.async_hooks events have an id and a special triggerId property.
• link to the output format specifications

ref: #15538 (comment)