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 7679cb3 reworking the exisitng information in the
process.

docs/libcurl/opts/CURLOPT_USERPWD.3

@@ -31,10 +31,21 @@ CURLcode curl_easy_setopt(CURL *handle, CURLOPT_USERPWD, char *userpwd);
 Pass a char * as parameter, pointing to a zero terminated login details string
 for the connection. The format of which is: [user name]:[password].
 
-When using NTLM, you can set the domain by prepending it to the user name and
-separating the domain and name with a forward (/) or backward slash (\\). Like
-this: "domain/user:password" or "domain\\user:password". Some HTTP servers (on
-Windows) support this style even for Basic authentication.
+When using Kerberos V5 authentication with a Windows based server, you should
+specify the user name part with the domain name in order for the server to
+successfully obtain a Kerberos Ticket. If you don't then the initial part of
+the authentication handshake may fail.
+
+When using NTLM, the user name can be specified simply as the user name
+without the domain name should the server be part of a single domain and
+forest.
+
+To specify the domain name use either Down-Level Logon Name or UPN (User
+Principal Name) formats. For example, EXAMPLE\user and user@example.com
+respectively.
+
+Some HTTP servers (on Windows) support inclusion of the domain for Basic
+authentication as well.
 
 When using HTTP and \fICURLOPT_FOLLOWLOCATION(3)\fP, libcurl might perform
 several requests to possibly different hosts. libcurl will only send this user