Add explicit "golang:alpine" variant description

[tianon]

This adds some Go-specific details, including links to golang/go#19938 and docker-library/golang#250 (comment).

Diff:

$ git diff --no-index .template-helpers/variant-alpine.md golang/variant-alpine.md
diff --git a/.template-helpers/variant-alpine.md b/golang/variant-alpine.md
index c94e7180f..f14e58f4f 100644
--- a/.template-helpers/variant-alpine.md
+++ b/golang/variant-alpine.md
@@ -2,6 +2,8 @@
 
 This image is based on the popular [Alpine Linux project](https://alpinelinux.org), available in [the `alpine` official image](https://hub.docker.com/_/alpine). Alpine Linux is much smaller than most distribution base images (~5MB), and thus leads to much slimmer images in general.
 
-This variant is highly recommended when final image size being as small as possible is desired. The main caveat to note is that it does use [musl libc](https://musl.libc.org) instead of [glibc and friends](https://www.etalabs.net/compare_libcs.html), so certain software might run into issues depending on the depth of their libc requirements. However, most software doesn't have an issue with this, so this variant is usually a very safe choice. See [this Hacker News comment thread](https://news.ycombinator.com/item?id=10782897) for more discussion of the issues that might arise and some pro/con comparisons of using Alpine-based images.
+This variant is highly experimental, and *not* officially supported by the Go project (see [golang/go#19938](https://github.com/golang/go/issues/19938) for details).
 
-To minimize image size, it's uncommon for additional related tools (such as `git` or `bash`) to be included in Alpine-based images. Using this image as a base, add the things you need in your own Dockerfile (see the [`alpine` image description](https://hub.docker.com/_/alpine/) for examples of how to install packages if you are unfamiliar).
+The main caveat to note is that it does use [musl libc](https://musl.libc.org) instead of [glibc and friends](https://www.etalabs.net/compare_libcs.html), which can lead to unexpected behavior. See [this Hacker News comment thread](https://news.ycombinator.com/item?id=10782897) for more discussion of the issues that might arise and some pro/con comparisons of using Alpine-based images.
+
+To minimize image size, additional related tools (such as `git`, `gcc`, or `bash`) are not included in Alpine-based images. Using this image as a base, add the things you need in your own Dockerfile (see the [`alpine` image description](https://hub.docker.com/_/alpine/) for examples of how to install packages if you are unfamiliar). See also [docker-library/golang#250 (comment)](https://github.com/docker-library/golang/issues/250#issuecomment-451201761) for a longer explanation.

Closes docker-library/golang#153