Adding executor key to job reference table (DOCSS-1956) (#9718)

* Adding  key to job reference table

* Marking key as required

Author: yaningo

docs/reference/modules/ROOT/pages/configuration-reference.adoc

@@ -341,6 +341,11 @@ Each job consists of the job's name as a key and a map as a value. A name should
 | Map
 | Options for the <<macos,macOS executor>>
 
+| `executor`
+| Y ^(1)^
+| String
+| Name of a declared <<executors,reusable executor>>
+
 | `shell`
 | N
 | String