Add finallystarttime to PipelineRun status docs

This commit adds details on `finallyStartTime` to PipelineRun documentation.

Author: lbernick

docs/pipelineruns.md

@@ -1387,6 +1387,8 @@ Your `PipelineRun`'s `status` field can contain the following fields:
     - [`apiVersion`][kubernetes-overview] - The API version for the underlying `TaskRun` or `Run`.
     - [`whenExpressions`](pipelines.md#guard-task-execution-using-when-expressions) - The list of when expressions guarding the execution of this task.
   - `provenance` - Metadata about resources used in the PipelineRun such as the source from where a remote pipeline definition was fetched.
+  - `finallyStartTime`- The time at which the PipelineRun's `finally` Tasks, if any, began
+  executing, in [RFC3339](https://tools.ietf.org/html/rfc3339) format.
 
 ### Configuring usage of `TaskRun` and `Run` embedded statuses