pveum: active directory: document user attribute name option

AD realms support the 'user_attr' option like LDAP realms, but it is
only documented for the latter. Describe it with its default and the
use case of logging in with the user principal name.

Signed-off-by: Michal Fox <me@dualfroz.com>

pveum.adoc

@@ -288,6 +288,11 @@ The main configuration settings for Microsoft Active Directory are:
 
 * `Domain` (`domain`): The AD domain of the server
 
+* `User Attribute Name` (`user_attr`): The AD attribute containing the
+  username that users will log in with, `sAMAccountName` by default. Set it to
+  `userPrincipalName` to let users log in with their user principal name, for
+  example `user@example.com`.
+
 * `Server` (`server1`): The FQDN or IP address of the server
 
 * `Fallback Server` (`server2`): An optional fallback server address, in case