[kots]: note to explain the region limitation on registry S3 backing

When creating an S3 backing to the in-cluster container registry, the
`s3.amazonaws.com` endpoint only works if the bucket is in the
`us-east-1` region. Without it, the registry healthcheck eventually
reports a 503 error.

This documents the requirement in the KOTS dashboard.

Author: Simon Emms

install/kots/manifests/kots-config.yaml

@@ -58,7 +58,7 @@ spec:
           required: true
           value: s3.amazonaws.com
           when: '{{repl and (ConfigOptionEquals "reg_incluster" "1") (ConfigOptionEquals "reg_incluster_storage" "s3") }}'
-          help_text: The endpoint used to connect to the S3 storage.
+          help_text: The endpoint used to connect to the S3 storage. If not using the `us-east-1` region, this should be `s3.<region>.amazonaws.com`.
 
         - name: reg_incluster_storage_s3_bucketname
           title: S3 bucket name