async_hooks: clean up comments

With some of the recent work, some of the comments were no longer
representative of the code, or were otherwise unclear. This commit
fixes some obvious issues I found.

Ref: https://github.com/nodejs/node/commit/83e5215a4e8438a43b9f0002b7a43e2fd2dd37a4
Ref: https://github.com/nodejs/node/commit/0784b0440c05464f79b857f7d8698fcc953d3fb3
PR-URL: https://github.com/nodejs/node/pull/18467


Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>