cloud/storage: add comprehensive docs about the signed URL private keys
cloud/storage: add comprehensive docs about the signed URL private keys

Change-Id: I821fdc26d3c9c7438dce1d68f11e76a53681f745

googleapis/google-cloud-go,roth1002/gcloud-golang,roth1002/gcloud-golang,michael-grosshans/google-cloud-go,Maksadbek/gcloud-golang,michael-grosshans/google-cloud-go,blowmage/gcloud-golang,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,Maksadbek/gcloud-golang,ptomasroos/gcloud-golang,hsinhoyeh/gcloud-golang,hsinhoyeh/gcloud-golang,ptomasroos/gcloud-golang,blowmage/gcloud-golang