add comments for claims module
+//! Credential Claims module.
+//!
+//! Contains constants used as keys into the [AwsCredentials](../struct.AwsCredentials.html) claims map
+//! returned by the STS client's federated assume role APIs.
+/// Key used in the claims map for the `Subject` claim.
+///
+/// The `Subject` is the unique user identifier that is returned by the identity provider.
+/// For SAML this is the `NameID` element in the `Subject` element of the SAML assertion.
+/// For OpenID Connect this field contains the value returned by the identity provider
+/// as the token's `sub` claim.
pub const SUBJECT: &'static str = "sub";
+
+/// Key used in the claims map for the `Audience` claim.
+///
+/// The intended audience (also known as client ID) of the web identity token.
+/// This is traditionally the client identifier issued to the application that requested the web identity token.
+/// For OpenID Connect this field contains the value of the `aud` claim.
+/// For SAML this is the value of the `Recipient` attribute of the `SubjectConfirmationData`
+/// element of the SAML assertion.
pub const AUDIENCE: &'static str = "aud";
+
+/// Key used in the claims map for the `Issuer` claim.
+///
+/// For OpenID Connect ID Tokens this contains the value of the `iss` field.
+/// For OAuth 2.0 access tokens, this contains the value of the `ProviderId` parameter
+/// that was passed in the `AssumeRoleWithWebIdentity` request.
+/// For SAML this is the value of the Issuer element of the SAML assertion.
pub const ISSUER: &'static str = "iss";
\ No newline at end of file
