docs: Document ApplyOutOfSyncOnly in application spec (argoproj#16282)

I found this being used in an application I was working on but had a
hard time discovering its meaning since it wasn't in the
specification[1] though it was in the `sync-options` docs[2] (see also
baa0f2e)

[1] https://argo-cd.readthedocs.io/en/stable/user-guide/application-specification/
[2] https://argo-cd.readthedocs.io/en/stable/user-guide/sync-options/#selective-sync

Signed-off-by: Matt Hughes <mhughes@uw.co.uk>

docs/operator-manual/application.yaml

@@ -189,6 +189,7 @@ spec:
     - PrunePropagationPolicy=foreground # Supported policies are background, foreground and orphan.
     - PruneLast=true # Allow the ability for resource pruning to happen as a final, implicit wave of a sync operation
     - RespectIgnoreDifferences=true # When syncing changes, respect fields ignored by the ignoreDifferences configuration
+    - ApplyOutOfSyncOnly=true # Only sync out-of-sync resources, rather than applying every object in the application
     managedNamespaceMetadata: # Sets the metadata for the application namespace. Only valid if CreateNamespace=true (see above), otherwise it's a no-op.
       labels: # The labels to set on the application namespace
         any: label