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

Checklist

• commit message follows commit guidelines

Affected core subsystem(s)

async_hooks