CURLOPT_USERPWD.3: Updated following Kerberos V5 SSPI changes

Added information about Kerberos V5 requiring the domain part in the
user name.

Mentioned that the user name can be specified in UPN format, and not
just in Down-Level Logon Name format, following the information
added in commit 7679cb3f reworking the exisitng information in the
process.