Add blurb for "*-stretch" and "*-xenial" tags; Improve variant detection for "windowsservercore-zzz" tags

[yosifkit]

Fixes docker-library/python#343

Python changes:

@@ -180,7 +180,11 @@ The `python` images come in many flavors, each designed for a specific use case.
 
 ## `python:<version>`
 
-This is the defacto image. If you are unsure about what your needs are, you probably want to use this one. It is designed to be used both as a throw away container (mount your source code and start the container to start your app), as well as the base to build other images off of. This tag is based off of [`buildpack-deps`](https://registry.hub.docker.com/_/buildpack-deps/). `buildpack-deps` is designed for the average user of docker who has many images on their system. It, by design, has a large number of extremely common Debian packages. This reduces the number of packages that images that derive from it need to install, thus reducing the overall size of all images on your system.
+This is the defacto image. If you are unsure about what your needs are, you probably want to use this one. It is designed to be used both as a throw away container (mount your source code and start the container to start your app), as well as the base to build other images off of.
+
+This tag is based off of [`buildpack-deps`](https://registry.hub.docker.com/_/buildpack-deps/). `buildpack-deps` is designed for the average user of docker who has many images on their system. It, by design, has a large number of extremely common Debian packages. This reduces the number of packages that images that derive from it need to install, thus reducing the overall size of all images on your system.
+
+Some of these tags may have names like jessie, stretch, or wheezy in them. These are the suite code names for releases of [Debian](https://wiki.debian.org/DebianReleases) and indicate which release the image is based on.
 
 ## `python:<version>-slim`

@@ -194,6 +198,15 @@ This variant is highly recommended when final image size being as small as possi
 
 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).
 
+## `python:<version>-windowsservercore`
+
+This image is based on [Windows Server Core (`microsoft/windowsservercore`)](https://hub.docker.com/r/microsoft/windowsservercore/). As such, it only works in places which that image does, such as Windows 10 Professional/Enterprise (Anniversary Edition) or Windows Server 2016.
+
+For information about how to get Docker running on Windows, please see the relevant "Quick Start" guide provided by Microsoft:
+
+-      [Windows Server Quick Start](https://msdn.microsoft.com/en-us/virtualization/windowscontainers/quick_start/quick_start_windows_server)
+-      [Windows 10 Quick Start](https://msdn.microsoft.com/en-us/virtualization/windowscontainers/quick_start/quick_start_windows_10)
+

[tianon]

While we're here, can we drop registry. from this URL? 😇

[tianon]

(and capitalize "docker")

[tianon]

I got the rest of the registry. references in c06ef55 👍