update variables.md to reflect the behavior after tektoncd#1522 is re…

…solved.

docs/variables.md

@@ -14,6 +14,7 @@ This page documents the variable substitions supported by `Tasks` and `Pipelines
 | -------- | ----------- |
 | `params.<param name>` | The value of the parameter at runtime. |
 | `tasks.<taskName>.results.<resultName>` | The value of the `Task's` result. Can alter `Task` execution order within a `Pipeline`.) |
+| `context.pipelineRun.name` | The name of the `PipelineRun` that this `Pipeline` is running in. |
 
 ## Variables available in a `Task`
 
@@ -27,6 +28,7 @@ This page documents the variable substitions supported by `Tasks` and `Pipelines
 | `workspaces.<workspaceName>.claim` | The name of the `PersistentVolumeClaim` specified as a volume source for the `Workspace`. Empty string for other volume types. |
 | `workspaces.<workspaceName>.volume` | The name of the volume populating the `Workspace`. |
 | `credentials.path` | The path to the credentials written by the `creds-init` init container. |
+| `context.taskRun.name` | The name of the `TaskRun` that this `Task` is running in. |
 
 ### `PipelineResource` variables available in a `Task`