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.

PR-URL: https://github.com/nodejs/node/pull/28100


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>