Remove 'signed assertions about the user'

Given FedCM is unopinionated about what information a token carries, 'The content of the token ... can contain anything that the IDP would like to pass to the RP to facilitate the login'. Maybe this description should not say it will contain signed user assertions.

spec/index.bs

@@ -1999,7 +1999,7 @@ For example:
 ## Identity assertion endpoint ## {#idp-api-id-assertion-endpoint}
 <!-- ============================================================ -->
 
-The <dfn>identity assertion endpoint</dfn> is responsible for minting a new token that contains signed assertions about the user.
+The <dfn>identity assertion endpoint</dfn> is responsible for minting a new token.
 
 The [=identity assertion endpoint=] is fetched in the [=fetch an identity assertion=] algorithm: