[doc] add mimetype.use-xattr to create-mime.conf.pl

x-ref:
  "Migrate to freedesktop.org definition of xattr mimetype"
  https://redmine.lighttpd.net/issues/2631

doc/scripts/create-mime.conf.pl

@@ -150,8 +150,19 @@ print <<EOF;
 ## https://redmine.lighttpd.net/projects/lighttpd/wiki/Mimetype_assignDetails
 
 ##
-## Use the "Content-Type" extended file attribute to obtain mime type if
-## possible
+## mimetype.xattr-name
+## Set the extended file attribute name used to obtain mime type
+## (must also set mimetype.use-xattr = "enable")
+##
+## Default value is "Content-Type"
+##
+## freedesktop.org Shared MIME-info Database specification suggests
+## user-defined value ("user.mime_type") as name for extended file attribute
+#mimetype.xattr-name = "user.mime_type"
+
+##
+## Use extended attribute named in mimetype.xattr-name (default "Content-Type")
+## to obtain mime type if possible
 ##
 ## Disabled by default
 ##