doc: update Concurrency, Functions and Methods, and Control Flow sections
doc: update Concurrency, Functions and Methods, and Control Flow sections

Many parts of the FAQ are dusty and need cleaning up.
This is the first of a series of changes to bring it up to date.
Since it was first written at the time of release, some of the
ideas and background have changed, and some historical
remarks are largely irrelevant now.

Update #26107.

Change-Id: I1f36df7d8ecc8a1a033d5ac4fa1edeece25ed6b4
Reviewed-on: https://go-review.googlesource.com/123496
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,cloudflare/go,josharian/go,nawawi/go,josharian/go,akutz/go,cloudflare/go,akutz/go,golang/go,akutz/go,golang/go,josharian/go,golang/go,CAFxX/go,akutz/go,nawawi/go,golang/go,akutz/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,golang/go,josharian/go,CAFxX/go,CAFxX/go,nawawi/go,josharian/go,josharian/go,cloudflare/go,CAFxX/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go