DOCS: Spread stanza does not exist on task

Fixes documentation inaccuracy for spread stanza placement. Spreads can
only exist on the top level job struct or within a group.

comment about nil assumption

scheduler/util.go

@@ -337,6 +337,8 @@ func shuffleNodes(nodes []*structs.Node) {
 // tasksUpdated does a diff between task groups to see if the
 // tasks, their drivers, environment variables or config have updated. The
 // inputs are the task group name to diff and two jobs to diff.
+// taskUpdated and functions called within assume that the given
+// taskGroup has already been checked to not be nil
 func tasksUpdated(jobA, jobB *structs.Job, taskGroup string) bool {
 	a := jobA.LookupTaskGroup(taskGroup)
 	b := jobB.LookupTaskGroup(taskGroup)

website/source/docs/job-specification/spread.html.md

@@ -17,8 +17,6 @@ description: |-
       <code>job -> **spread**</code>
       <br>
       <code>job -> group -> **spread**</code>
-      <br>
-      <code>job -> group -> task -> **spread**</code>
     </td>
   </tr>
 </table>