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 <iant@golang.org>