Manual: define the “DATE” placeholder

[skip ci]

doc/manual/source/the-model-file.rst

@@ -98,7 +98,10 @@ observed:
   system; it is necessary to put it between quotes when specifying the
   extension or if the filename contains a non-alphanumeric character;
 * QUOTED_STRING indicates an arbitrary string enclosed between (single)
-  quotes.
+  quotes;
+* DATE indicates a time period which can be either a year (e.g. `2024Y` or
+  `2024A`), a half-year (`2024S1` or `2024H1`), a quarter (`2024Q2`) or a month
+  (`2024M3`) (see :ref:`dates in a mod file`).
 .. _var-decl: