Enhancement - #390 timestamps in annotations

- added new dateformat for annotations
- documented prev. added format modifiers H, N and S

doc/man/taskrc.5

@@ -285,6 +285,8 @@ for example, or 'export'.
 .TP
 .B dateformat.holiday=YMD
 .TP
+.B dateformat.annotation=m/d/Y
+.TP
 .B report.X.dateformat=m/d/Y
 This is a string of characters that define how task formats date values. The
 precedence order for the configuration variable is report.X.dateformat then
@@ -319,6 +321,12 @@ b  short name of month,   for example Jan or Aug
 B  long name of month,    for example January or August
 .br
 V  weeknumber,            for example 03 or 37
+.br
+H  two-digit hour,        for example 03 or 11
+.br
+N  two-digit minutes,     for example 05 or 42
+.br
+S  two-digit seconds,     for example 07 or 47
 .RE
 .RE