Add relative date documentation to manpage

src/include/doxygen.h

  *@par -v[v[v[...]]]
  *Augment the loglevel
  *
- *@section datefmt Date format
- * The dateformat understood by ttail is the strptime date format :
+ *@section datefmt Date formats
  *
+ * ttail understand two date formats : strprtime and relative
+ *
+ *@subsection datefmt_rel Relative date format
+ *Relative date format is used with -d --date-min or -m --date-max arguments.
+ *The shape of a relative date is "-#[VAL][UNIT]" with VAL an integer and UNIT
+ *one of the recognized units :
+ *- y[ear]
+ *- M[onth]
+ *- d[ay]
+ *- h[our]
+ *- m[in]
+ *- s[ec]
+ *
+ *@subsection datefmt_strptime Strptime date format
  * The supported input field descriptors are listed below.  In case a text string (such as the name
  * of a day of the week or a month name) is to be matched, the comparison is case insensitive.   In
  * case a number is to be matched, leading zeros are permitted but not required.