Update documentation to be clearer about where to add auto injection annotations

[charlie-hadden]

When I was setting linkerd2 up for the first time I added my inkerd.io/inject: enabled annotation to the deployment, and I was seeing the injector skipping it. After discussing on slack I was pointed in the right direction and moved it pod template where things worked correctly.

However, the documentation in a couple of places seems to suggest that my original way would have worked correctly. The wording which was throwing me off was:

Linkerd automatically adds the data plane proxy to pods when the linkerd.io/inject: enabled annotation is present on a namespace, deployment, or pod.

from here: https://linkerd.io/2/features/proxy-injection/

This is typically done by annotating the namespace, deployment, or pod with the linkerd.io/inject: enabled Kubernetes annotation, which will trigger automatic proxy injection when the resources are created.

from here: https://linkerd.io/2/tasks/adding-your-service/

[alpeb]

Also now that linkerd/linkerd2#3194 has merged, we should point out that these annotations are also available at the namespace level