Add workspace option in documentation for GCP data remote state

[chrismackmn]

The documentation on this is really poor: https://www.terraform.io/docs/providers/terraform/d/remote_state.html

Nowhere on this page does it say to configure a workspace.

Originally posted by @gswallow in #17153 (comment)

Please add workspace in documentation for gcs data backends. This wasn't clear in the documentation.

https://www.terraform.io/docs/backends/types/gcs.html

Need to add
+ defaults = null
+ outputs = {}
+ workspace = null

so everything is in the documentation.

Changes to Outputs:
~ blah = {
+ backend = "gcs"
+ config = {
+ bucket = "mystate-bucket"
+ credentials = "secrets-file.json"
+ prefix = "admin"
}
+ defaults = null
+ outputs = {}
+ workspace = null
}