tools: clarify that qvm-backup needs absolute path

Fixes QubesOS/qubes-issues#3066

doc/manpages/qvm-backup.rst

@@ -66,9 +66,9 @@ Options
 Arguments
 ---------
 
-The first positional parameter is the backup location (directory path, or
-command to pipe backup to). After that you may specify the qubes you'd like to
-backup. If not specified, all qubes are included.
+The first positional parameter is the backup location (absolute directory path,
+or command to pipe backup to). After that you may specify the qubes you'd
+like to backup. If not specified, all qubes are included.
 
 Authors
 -------

qubesadmin/tools/qvm_backup.py

@@ -77,7 +77,8 @@ no_profile.add_argument('--save-profile', action='store',
 
 no_profile.add_argument("backup_location", action="store", default=None,
     nargs='?',
-    help="Backup location (directory path, or command to pipe backup to)")
+    help="Backup location (absolute directory path, "
+         "or command to pipe backup to)")
 
 no_profile.add_argument("vms", nargs="*", action=qubesadmin.tools.VmNameAction,
     help="Backup only those VMs")