Update gcp auth backend docs (#3209)
Update gcp auth backend docs (#3209)

* Update gcp auth backend docs

* Minor formatting and wording fixes

* Minor formatting fixes

deverton/hashicorp-vault,rnaveiras/vault,joelthompson/vault,Aloomaio/vault,kingland/vault,lemondrank/vault,mgk/vault,hashicorp/vault,hashicorp/vault,wjam/vault,rnaveiras/vault,Aloomaio/vault,rnaveiras/vault,mgk/vault,joelthompson/vault,quixoten/vault,naunga/vault,hashicorp/vault,hartsock/vault,rnaveiras/vault,hartsock/vault,Aloomaio/vault,tallpauley/vault,hashicorp/vault,hartsock/vault,tallpauley/vault,Aloomaio/vault,hashicorp/vault,deverton/hashicorp-vault,quixoten/vault,quixoten/vault,joelthompson/vault,joelthompson/vault,kingland/vault,lemondrank/vault,mgk/vault,hartsock/vault,kingland/vault,mgk/vault,quixoten/vault,naunga/vault,hashicorp/vault,hartsock/vault,rnaveiras/vault,kingland/vault,kingland/vault,wjam/vault