docs(designs): update runbook.md

ref: #1212 (comment)

Author: k1LoW

docs/designs/runbook.md

@@ -18,3 +18,5 @@ runn developers need to have a common understanding of the properties of the run
 
 - The runbook is the minimum unit of running of runn. For example, it is not assumed to have features that run only certain steps in a runbook.
 - The running of the steps in a runbook is always sequential. Concurrent running is not assumed.
+- All steps that have started running will stop when that runbook is completed (e.g. `exec.background:`).
+    - Steps marked with `defer:` have not started running, so run execution is delayed until completion of the parent runbook.