OCPBUGS-10984: Document the --skip-pruning oc-mirror flag

Author: bergerhoffer

modules/oc-mirror-command-reference.adoc

@@ -92,6 +92,9 @@ The following tables describe the `oc mirror` subcommands and flags:
 |`--skip-missing`
 |If an image is not found, skip it instead of reporting an error and aborting execution. Does not apply to custom images explicitly specified in the image set configuration.
 
+|`--skip-pruning`
+|Disable automatic pruning of images from the target mirror registry.
+
 |`--skip-verification`
 |Skip digest verification.
 

modules/oc-mirror-updating-registry-about.adoc

@@ -34,3 +34,5 @@ If there are {product-title} releases or Operator versions that you no longer ne
 ====
 If there are no new or updated images to mirror, the excluded images are not pruned from the target mirror registry. Additionally, if an Operator publisher removes an Operator version from a channel, the removed versions are pruned from the target mirror registry.
 ====
+
+To disable automatic pruning of images from the target mirror registry, pass the `--skip-pruning` flag to the `oc mirror` command.