Update usage w.r.t. numeric ionice priority classes

Author: xolox

README.rst

@@ -193,8 +193,9 @@ intended you have no right to complain ;-).
      use the ``-r``, ``--relaxed`` option in order to preserve more backups."
    "``-i``, ``--ionice=CLASS``","Use the ""ionice"" program to set the I/O scheduling class and priority of
    the ""rm"" invocations used to remove backups. ``CLASS`` is expected to be one of
-   the values ""idle"", ""best-effort"" or ""realtime"". Refer to the man page of
-   the ""ionice"" program for details about these values."
+   the values ""idle"" (3), ""best-effort"" (2) or ""realtime"" (1). Refer to the
+   man page of the ""ionice"" program for details about these values. The
+   numeric values are required by the 'busybox' implementation of 'ionice'."
    "``-c``, ``--config=FILENAME``","Load configuration from ``FILENAME``. If this option isn't given the following
    default locations are searched for configuration files:
 

rotate_backups/cli.py

@@ -126,8 +126,9 @@
 
     Use the `ionice' program to set the I/O scheduling class and priority of
     the `rm' invocations used to remove backups. CLASS is expected to be one of
-    the values `idle', `best-effort' or `realtime'. Refer to the man page of
-    the `ionice' program for details about these values.
+    the values `idle' (3), `best-effort' (2) or `realtime' (1). Refer to the
+    man page of the `ionice' program for details about these values. The
+    numeric values are required by the 'busybox' implementation of 'ionice'.
 
   -c, --config=FILENAME