Add TIMESPAN to doc

- Closes borgbackup/borg#8310

docs/usage/general/date-time.rst.inc

@@ -8,3 +8,10 @@ For more information about that, see: https://xkcd.com/1179/
 
 Unless otherwise noted, we display local date and time.
 Internally, we store and process date and time as UTC.
+
+
+.. rubric:: TIMESPAN
+
+Some options accept a TIMESPAN parameter, which can be given as a
+number of days (e.g. ``7d``) or months (e.g. ``12m``).
+