Lack of documentation about DisableGardenerServiceAccountCreation usage

[donistz]

How to categorize this issue?

/area security
/kind bug
/platform gcp

What happened:
GCP extension documentation https://github.com/gardener/gardener-extension-provider-gcp/blob/master/docs/usage-as-end-user.md doesn't reflect this feature: #451
What you expected to happen:
It must be described in the documentation what permissions are necessary when DisableGardenerServiceAccountCreation is used.
https://github.com/gardener/gardener-extension-provider-gcp/blob/master/docs/usage-as-end-user.md to reproduce it (as minimally and precisely as possible)**:

Anything else we need to know?:

Environment:

• Gardener version (if relevant):
• Extension version:
• Kubernetes version (use kubectl version):
• Cloud provider or hardware configuration:
• Others:

[gardener-robot]

@donistz Label area/todo does not exist.