doc: update Effective Go with 1.14 language changes
doc: update Effective Go with 1.14 language changes

Fixes #37560

Change-Id: Iccb8e53254c45d203c1b42ea9b4d8509b93dd7a9
GitHub-Last-Rev: 5972b67e5d7b3b36ce1854ee9365197e78f654cd
GitHub-Pull-Request: golang/go#37563
Reviewed-on: https://go-review.googlesource.com/c/go/+/221429
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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