add remediation for AADSTS51004

previous commit was missing remediation from AADSTS51004

Author: nickludwig

articles/active-directory/develop/reference-aadsts-error-codes.md

@@ -187,7 +187,7 @@ The `error` field has several possible values - review the protocol documentatio
 | AADSTS51000 | RequiredFeatureNotEnabled - The feature is disabled. |
 | AADSTS51001 | DomainHintMustbePresent - Domain hint must be present with on-premises security identifier or on-premises UPN. |
 | AADSTS1000104| XCB2BResourceCloudNotAllowedOnIdentityTenant - Resource cloud {resourceCloud} isn't allowed on identity tenant {identityTenant}. {resourceCloud} - cloud instance which owns the resource. {identityTenant} - is the tenant where signing-in identity is originated from. |
-| AADSTS51004 | UserAccountNotInDirectory - The user account doesn’t exist in the directory. |
+| AADSTS51004 | UserAccountNotInDirectory - The user account doesn’t exist in the directory. An application likely chose the wrong tenant to sign into, and the currently logged in user was prevented from doing so since they did not exist in your tenant. If this user should be able to log in, add them as a guest. For further information, please visit [add B2B users](/azure/active-directory/b2b/add-users-administrator). |
 | AADSTS51005 | TemporaryRedirect - Equivalent to HTTP status 307, which indicates that the requested information is located at the URI specified in the location header. When you receive this status, follow the location header associated with the response. When the original request method was POST, the redirected request will also use the POST method. |
 | AADSTS51006 | ForceReauthDueToInsufficientAuth - Integrated Windows authentication is needed. User logged in using a session token that is missing the integrated Windows authentication claim. Request the  user to log in again. |
 | AADSTS52004 | DelegationDoesNotExistForLinkedIn - The user has not provided consent for access to LinkedIn resources. |