tokenserver: Fix some mistakes in proto docs for new token grants feature.

tokenserver/api/oauth_token_grant.pb.go

 // Code generated by protoc-gen-go. DO NOT EDIT.
 // source: github.com/luci/luci-go/tokenserver/api/oauth_token_grant.proto
 
 package tokenserver
 
 import proto "github.com/golang/protobuf/proto"
 import fmt "fmt"
 import math "math"
 import google_protobuf "github.com/golang/protobuf/ptypes/timestamp"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
 var _ = fmt.Errorf
 var _ = math.Inf
 
 // OAuthTokenGrantBody contains the internal guts of an oauth token grant.
 //
 // It gets serialized, signed and stuffed into OAuthTokenGrantEnvelope, which
 // then also gets serialized to get the final blob with the grant. This blob is
 // then base64-encoded and returned to the caller of MintOAuthTokenGrant.
 type OAuthTokenGrantBody struct {
         // Identifier of this token as generated by the token server.
         //
         // Used for logging and tracking purposes.
         //
         // TODO(vadimsh): It may later be used for revocation purposes.
         TokenId int64 `protobuf:"varint,1,opt,name=token_id,json=tokenId" json:"token_id,omitempty"`
-        // Service account identity the end user wants to act as.
-        //
-        // A string of the form "user:<email>".
+        // Service account email the end user wants to act as.
         ServiceAccount string `protobuf:"bytes,2,opt,name=service_account,json=serviceAccount" json:"service_account,omitempty"`
         // Who can pass this token to MintOAuthTokenViaGrant to get an OAuth token.
         //
         // A string of the form "user:<email>". On Swarming, this is Swarming's own
         // service account name.
         Proxy string `protobuf:"bytes,3,opt,name=proxy" json:"proxy,omitempty"`
         // An end user that wants to act as the service account (perhaps indirectly).
         //
         // A string of the form "user:<email>". On Swarming, this is an identity of
         // a user that posted the task.
         //
-        // This is informational field currently (not used in authorization checks).
+        // Used by MintOAuthTokenViaGrant to recheck that the access is still allowed.
         EndUser string `protobuf:"bytes,4,opt,name=end_user,json=endUser" json:"end_user,omitempty"`
         // When the token was generated (and when it becomes valid).
         IssuedAt *google_protobuf.Timestamp `protobuf:"bytes,5,opt,name=issued_at,json=issuedAt" json:"issued_at,omitempty"`
         // How long the token is considered valid (in seconds).
         //
         // It may become invalid sooner if the token server policy changes and the
         // new policy doesn't allow this token.
         ValidityDuration int64 `protobuf:"varint,6,opt,name=validity_duration,json=validityDuration" json:"validity_duration,omitempty"`
 }
 
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 }