Issue w3c#294 - Add a non-normative comment about snapshotting Buffer…

…Sources

index.bs

@@ -958,6 +958,10 @@ This [=internal method=] accepts three arguments:
 Note: <strong>This algorithm is synchronous:</strong> the {{Promise}} resolution/rejection is handled by
 {{CredentialsContainer/create()|navigator.credentials.create()}}.
 
+Note: All {{BufferSource}} objects used in this algorithm must be snapshotted when the algorithm begins, to
+avoid potential synchronization issues. The algorithm implementations should [=get a copy of the bytes held
+by the buffer source=] and use that copy for relevant portions of the algorithm.
+
 When this method is invoked, the user agent MUST execute the following algorithm:
 
 1. Assert: <code>|options|.{{CredentialCreationOptions/publicKey}}</code> is [=present=].
@@ -1323,6 +1327,10 @@ This [=internal method=] accepts three arguments:
 Note: <strong>This algorithm is synchronous:</strong> the {{Promise}} resolution/rejection is handled by
 {{CredentialsContainer/get()|navigator.credentials.get()}}.
 
+Note: All {{BufferSource}} objects used in this algorithm must be snapshotted when the algorithm begins, to
+avoid potential synchronization issues. The algorithm implementations should [=get a copy of the bytes held
+by the buffer source=] and use that copy for relevant portions of the algorithm.
+
 When this method is invoked, the user agent MUST execute the following algorithm:
 
 1. Assert: <code>|options|.{{CredentialRequestOptions/publicKey}}</code> is [=present=].