Change the explanation of "default repo" setting in the doc to reflect actual implementation

Expected behavior

Describe that 3rd default-repo config option described in the doc depends on kubectl context, not global. https://skaffold.dev/docs/concepts/#image-repository-handling

Actual behavior

It's documented as "skaffold’s global config", which is misleading. (Though ~/.skaffold/config is global to all command call, the default repo setting is not "global")

Information

• Skaffold version: v0.20.0
• Operating system: Debian 9.6
• Contents of skaffold.yaml:

apiVersion: skaffold/v1beta2
kind: Config
build:
  artifacts:
  - image: frontendservice
    context: src/frontend
  - image: arrayparseservice
    context: src/arrayparse
  - image: addnumberservice
    context: src/addnumber
deploy:
  kubectl:
    manifests:
    - ./k8s-manifests/**.yaml
profiles:
- name: gcb
  build:
    googleCloudBuild:
      diskSizeGb: 30
      machineType: N1_HIGHCPU_32
      timeout: 4000s

Steps to reproduce the behavior

1. Set artifacts/image without target repo path.
2. Call skaffold config set default-repo <REPO> in some kubectl context
3. Change kubectl context
4. See the log The push refers to repository [docker.io/library/xxxxxx] and fail to deploy images during skaffold dev/run