doc: document trace-events category for dns requests
As implemented in https://github.com/nodejs/node/pull/21840, dns can emit trace events when the category is enabled. This PR just add it to the documentation.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines