Better man page entry for obnam diff

=== modified file 'obnam.1.in'
--- obnam.1.in  2012-09-29 13:33:13 +0000
+++ obnam.1.in  2012-10-10 22:05:07 +0000
@@ -41,6 +41,14 @@
 possibly followed by arguments.
 The commands are list below.
 .IP \(bu
+.B diff
+compares two generations and lists files differing between them. Every output
+line will be prefixed either by a plus sign (+) for files that were added, a
+minus sign (-) for files that have been removed or an asterisk (*) for files
+that have changed.  If only one generation ID is specified on the command line
+that generation will be compared with its direct predecessor. If two IDs have
+been specified, all changes between those two generations will be listed.
+.IP \(bu
 .B backup
 makes a new backup.
 The first time it is run, it makes a full backup,