Mention --force in --remove-older-than section

git-svn-id: http://svn.savannah.nongnu.org/svn/rdiff-backup@704 2b77aa54-bcbc-44c9-a7ec-4f6cf2b41109

trunk/rdiff-backup/rdiff-backup.1

@@ -378,8 +378,15 @@ hours, and 7 seconds.  In this context, a month means 30 days, a year
 is 365 days, and a day is always 86400 seconds.
 
 rdiff-backup cannot remove-older-than and back up or restore in a
-single session.  If you want to, for instance, backup a directory and
-remove old files in it, you must run rdiff-backup twice.
+single session.  In order to both backup a directory and remove old
+files in it, you must run rdiff-backup twice.
+
+By default, rdiff-backup will only delete information from one session
+at a time.  To remove two or more sessions at the same time, supply the
+.B --force 
+option (rdiff-backup will tell you if 
+.B --force
+is required).
 
 Note that snapshots of deleted files are covered by this operation.
 Thus if you deleted a file two weeks ago, backed up immediately