docs: update Docker descriptions (#2418)

website/docs/configuration-options.md

@@ -142,14 +142,20 @@ The description field is used by config presets to describe what they do. They a
 
 ## digest
 
-Add to this object if you wish to define rules that apply only to PRs that pin Docker digests.
+Add to this object if you wish to define rules that apply only to PRs that update Docker digests.
 
 ## docker
 
+Add config here if you wish it to apply to all Docker package managers (Dockerfile, Docker Compose, CircleCI, etc).
+
 ## docker-compose
 
+Add configuration here if you want to enable or disable something in particular for Docker Compose files and override the default Docker settings.
+
 ## dockerfile
 
+Add configuration here if you want to enable or disable something in particular for `Dockerfile` files and override the default Docker settings.
+
 ## enabled
 
 Renovate is enabled for all packages by default, but this setting allows you to disable Renovate for specific packages, dependency types, package files, or even for the whole repository.

website/docs/docker.md

@@ -51,6 +51,16 @@ Renovate has a some docker-specific intelligence when it comes to versions. For
 
 ## Configuring/Disabling
 
+If you with to make changes that apply to all Docker managers, then add them to the `docker` config object. If you wish to override Docker settings for one particular type of manager, use that manager's config object instead. For example. to disable digest updates for Docker Compose only but leave them for other managers like `Dockerfile`, you would add this:
+
+```json
+  "docker-compose": {
+    "digest": {
+      "enabled": false
+    }
+  }
+```
+
 The following configuration options are applicable to Docker:
 
 ##### Disable all Docker Renovation