docs(docker versioning): rewrite (#17097)

Co-authored-by: Rhys Arkins <rhys@arkins.net>
Co-authored-by: Jamie Magee <jamie.magee@gmail.com>

lib/modules/versioning/docker/readme.md

-Docker doesn't really have _versioning_, instead it supports "tags" and these are usually used by Docker image authors as a form of versioning.
+Docker images don't really have _versions_, instead they have "tags".
+Tags are often used by Docker image authors as a form of versioning.
 
-This Docker versioning implementation in Renovate is designed to handle the most common _conventions_ used in tagging images. In particular, it treats the text after the first hyphen as a type of platform/compatibility indicator.
+Renovate tries to follow the most common _conventions_ that are used to tag Docker images.
+In particular, Renovate treats the text after the first hyphen as a type of platform/compatibility indicator.
 
-For example, many images include images with the "-alpine" suffix, e.g. the official `node` Docker image includes tags like `12.15.0-alpine` which is _not_ compatible with `12.15.0` or `12.15.0-stretch`. This means users only want/expect upgrades to `12.16.0-alpine` and not `12.16.0` or `12.16.0-stretch`.
+For example, many images have releases with the `-alpine` suffix.
+The official `node` Docker image has tags like `12.15.0-alpine` which is _not_ compatible with `12.15.0` or `12.15.0-stretch`.
+Users on `-alpine` don't want updates to `12.16.0` or `12.16.0-stretch`.
+Those users only want upgrades to `12.16.0-alpine` and not `12.16.0` or `12.16.0-stretch`.
 
-Similarly, a user with `12.14` expects to be upgraded to `12.15` and not `12.15.0`.
+Similarly, a user on `12.14` expects to be upgraded to `12.15` and not `12.15.0`.
 
 **What type of versioning is used?**
 
-It's pretty "wild west" for tagging and not always compliant with SemVer. Docker versioning in Renovate should do a best effort to accept and sort SemVer-like versions.
+Docker image authors can use whatever tag they want, it's a "wild west".
+Docker tags don't always follow SemVer.
+This means that Renovate tries to accept and sort SemVer-like versions, but this won't always work.
+
+You may need to help Renovate and create your own rules for some Docker images.
+For example:
+
+```json
+{
+  "packageRules": [
+    {
+      "matchDatasources": ["docker"],
+      "matchPackageNames": ["badly-versioned-docker-image"],
+      "versioning": "loose"
+    }
+  ]
+}
+```
 
 **Are ranges supported?**
 
-No. Although a tag like `12.15` might seem like it means `12.15.x`, it is a tag of its own and may or may not point to an of the available `12.15.x` tags, including `12.15.0`.
+No.
+You may think a tag like `12.15` also means `12.15.x`, but it's a tag of its own.
+The `12.15` tag may or may not point to any of the available `12.15.x` tags, including `12.15.0`.
 
 **Are commit hashes supported?**
 
-No. An image tag that looks like a Git commit hash should be ignored by Renovate.
+No, Renovate ignores Docker image tags that look like a Git commit hash.