doc/faq: tweak the wording in the new section on goroutine IDs

doc/faq: tweak the wording in the new section on goroutine IDs

There were too many changes of direction. Tidy up the intro a little
for better flow, and delete some unnecessary comments.

Change-Id: Ib5d85c0992626bd3152f86a51585884d3e0cab72
Reviewed-on: https://go-review.googlesource.com/80495
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>