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

tokenserver/api/oauth_token_grant.proto

Index: tokenserver/api/oauth_token_grant.proto
diff --git a/tokenserver/api/oauth_token_grant.proto b/tokenserver/api/oauth_token_grant.proto
index f98052fe209431ef10c72acd4d3a4932fea2a609..b7cdefbfab78524057b59fd75fb582a69c4b524d 100644
--- a/tokenserver/api/oauth_token_grant.proto
+++ b/tokenserver/api/oauth_token_grant.proto
@@ -22,9 +22,7 @@ message OAuthTokenGrantBody {
   // TODO(vadimsh): It may later be used for revocation purposes.
   int64 token_id = 1;
 
-  // 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.

[Vadim Sh., 2017/08/04 05:55:53]

I decided all service account references (in configs and protos) should be just
emails, since they are not part of a data model we control (unlike "<kind>:<id>"
thingies called Identities). In other words, "user:" prefix is the only possible
prefix for service accounts, and so we can drop it.

   string service_account = 2;
 
   // Who can pass this token to MintOAuthTokenViaGrant to get an OAuth token.
@@ -38,7 +36,7 @@ message OAuthTokenGrantBody {
   // 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.

[Vadim Sh., 2017/08/04 05:55:53]

Realized that while writing MintOAuthTokenGrant implementation. 

   string end_user = 4;
 
   // When the token was generated (and when it becomes valid).