OCPBUGS 13867: Clarifying --manifests-only depends on --from

Author: bergerhoffer

modules/oc-mirror-command-reference.adoc

@@ -69,7 +69,7 @@ The following tables describe the `oc mirror` subcommands and flags:
 |Enable mirroring for local OCI catalogs on disk to the target mirror registry.
 
 |`--manifests-only`
-|Generate manifests for `ImageContentSourcePolicy` objects to configure a cluster to use the mirror registry, but do not actually mirror any images.
+|Generate manifests for `ImageContentSourcePolicy` objects to configure a cluster to use the mirror registry, but do not actually mirror any images. To use this flag, you must pass in an image set archive with the `--from` flag.
 
 |`--max-nested-paths <int>`
 |Specify the maximum number of nested paths for destination registries that limit nested paths. The default is `2`.