Message ZWEAG121E is 1) missing from doc 2) confusing/wrong.

Describe the bug
A clear and concise description of what the bug is.

I am trying to do certificate logon ( and no password ) to Zowe.

If a certificate has been mapped to a userid, it works.

If I provide a certificate which has not been mapped to a userid I get

{"messages":[{"messageType":"ERROR","messageNumber":"ZWEAG121E","messageContent":"Authorization header is missing,
or the request body is missing or invalid for URL '/zaas/api/v1/auth/login'","messageAction":"Provide valid authentication.","messageReason":"
The authorization header is missing, or the request body is missing or invalid.","messageKey":"org.zowe.apiml.security.login.invalidInput"}]}+

the statement The authorization header is missing, or the request body is missing or invalid is not true because it works with a mapped certificate.

Adding some text like
or the certificate could not be mapped to a userid. would help

Start by locating the implementation and documentation for message ZWEAG121E, then reproduce the certificate-logon case where the certificate is not mapped to a userid. Confirm the current behavior and update the message or its documentation so the unmapped-certificate case is accurately described; the relevant tests should pass with the corrected wording.