doc: update Values, Writing Code, and Pointers and Allocation sections of the FAQ
doc: update Values, Writing Code, and Pointers and Allocation sections of the FAQ

Significant surgery done to the Versioning section, bringing it closer to
modern thinking.

Also add a question about constants.

Update #26107.

Change-Id: Icf70b7228503c6baaeab0b95ee3e6bee921575aa
Reviewed-on: https://go-review.googlesource.com/123918
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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