Change the description of proxy cache when upstream tag is removed

Signed-off-by: stonezdj <stonezdj@gmail.com>

docs/administration/configure-proxy-cache/_index.md

@@ -34,7 +34,8 @@ The next time a user requests that image, Harbor checks the image's latest manif
 * If the image has not been updated in the target registry, the cached image is served from the proxy cache project.
 * If the image has been updated in the target registry, the new image is pulled from the target registry, then served and cached in the proxy cache project.
 * If the target registry is not reachable, the proxy cache project serves the cached image.
-* If the image is no longer in the target registry, no image is served.
+* If the image is no longer in the target registry, but the image is still in the proxy cache project, the image is served from the proxy cache project.
+
 
 As of Harbor v2.1.1, Harbor proxy cache fires a HEAD request to determine whether any layer of a cached image has been updated in the Docker Hub registry. Using this method to check the target registry will not trigger the [Docker Hub rate limiter](https://www.docker.com/blog/scaling-docker-to-serve-millions-more-developers-network-egress/). If any image layer was updated, the proxy cache will pull the new image, which will count towards the Docker Hub rate limiter.