Fix documentation to specify when revisions are created. (#3920)

- this should allign with the messages that are created within revisions https://github.com/cloudfoundry/cloud_controller_ng/blob/11ef2a547e6c50001a65e49855c159aecdb32381/app/actions/revision_resolver.rb#L60-L62
- we confirmed that revisions are not created when running tasks on a --tasks pushed application or when the app is pushed with the app is pushed with the task config

docs/v3/source/includes/resources/revisions/_header.md

@@ -1,14 +1,14 @@
 ## Revisions
 
-Revisions represent code used by an application at a specific time. The most recent revision for a running application represents code and configuration currently running in Cloud Foundry.
+Revisions represent code used by an application at a specific time. The most recent revision for a running application represents code and configuration currently running in Cloud Foundry. Revisions are not created for Tasks.
 
 Revision are created when the following is changed:
 
-* A new droplet is created for an app
+* A new droplet is deployed for an app
 
-* An app's environment variables are changed
+* An app is deployed with new environment variables
 
-* The custom start command for an app is added or changed
+* The app is deployed with a new or changed custom start command
 
 * An app rolls back to a prior revision