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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

Cofyc/go,codestation/go,golang/go,christopher-henderson/Go,cloudflare/go,josharian/go,Cofyc/go,CAFxX/go,golang/go,josharian/go,cloudflare/go,Cofyc/go,Cofyc/go,josharian/go,codestation/go,cloudflare/go,golang/go,josharian/go,josharian/go,CAFxX/go,codestation/go,codestation/go,nawawi/go,Cofyc/go,cloudflare/go,cloudflare/go,josharian/go,josharian/go,CAFxX/go,akutz/go,CAFxX/go,josharian/go,codestation/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,akutz/go,golang/go,Cofyc/go,akutz/go,cloudflare/go,akutz/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,Cofyc/go,akutz/go,golang/go,Cofyc/go,cloudflare/go,codestation/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,golang/go,codestation/go,nawawi/go,nawawi/go,codestation/go,nawawi/go,CAFxX/go