docs: document required target field for link-work-items safe output (#1488)

The link-work-items tool requires a 'target' field in its front-matter
configuration. Without it, Stage 3 execution fails with an error:
'link-work-items requires a target field in safe-outputs configuration'.

This was undocumented — comment-on-work-item and update-work-item both
have explicit 'Note: target is required' callouts, but link-work-items
had only an implicit hint via the YAML example. Add the explicit note
and clarify the YAML comment from 'Scoping policy' to 'Required'.

Verified against src/safe_outputs/link_work_items.rs lines 162-170.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

docs/safe-outputs.md

@@ -549,10 +549,12 @@ Links two Azure DevOps work items together.
 safe-outputs:
   link-work-items:
     allowed-link-types: []       # Optional — restrict which link types are allowed (empty = all)
-    target: "*"                  # Scoping policy (same as comment-on-work-item target)
+    target: "*"                  # Required — "*" allows any work item ID, or set to a specific ID
     max: 5                       # Maximum per run (default: 5)
 ```
 
+**Note:** The `target` field is required. If omitted, Stage 3 execution fails with an error. Use the same scoping options as `comment-on-work-item`: `"*"` for any work item, a numeric ID for a specific item, a list of IDs, or an area path prefix string.
+
 ### queue-build
 Queues an Azure DevOps pipeline build by definition ID.