--dry-run=true and --validate=true results in no validation performed

[cflewis]

/kind bug

What happened:
Attempting to run --dry-run=true and --validate=true reported that the object was created.

What you expected to happen:
The object should be rejected.

How to reproduce it (as minimally and precisely as possible):
Using the same files as in the documentation:

# Create the resource
% kubectl create -f resourcedefinition.yaml 
customresourcedefinition.apiextensions.k8s.io/crontabs.stable.example.com created

# This is a rejection as expected
% kubectl create -f my-crontab.yaml 
The CronTab "my-new-cron-object" is invalid: []: Invalid value: map[string]interface {}{"apiVersion":"stable.example.com/v1", "kind":"CronTab", "metadata":map[string]interface {}{"uid":"f671f1f9-9b59-11e8-ba9a-42010a8a0fe0", "selfLink":"", "clusterName":"", "name":"my-new-cron-object", "namespace":"default", "creationTimestamp":"2018-08-08T22:25:32Z"}, "spec":map[string]interface {}{"cronSpec":"* * * *", "image":"my-awesome-cron-image", "replicas":15}}: validation failure list:
spec.cronSpec in body should match '^(\d+|\*)(/\d+)?(\s+(\d+|\*)(/\d+)?){4}$'
spec.replicas in body should be less than or equal to 10

# This is a create which should be rejected, but instead claims it was created, because `--dry-run` was set to true.
% kubectl create --validate=true --dry-run=true --filename=my-crontab.yaml
crontab.stable.example.com/my-new-cron-object created (dry run)

Environment:

• Kubernetes version (use kubectl version): 1.11
• Cloud provider or hardware configuration: Google Kubernetes Engine
• OS (e.g. from /etc/os-release): Debian
• Kernel (e.g. uname -a): GNU/Linux x86_64

[k8s-ci-robot]

@cflewis: There are no sig labels on this issue. Please add a sig label.

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[cflewis]

Closing as a dupe of #64830, but it does seem documentation is required here. The confusion is very real!