manual: updated info on auxiliary variables

Related to Dynare/dynare#1811

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

@@ -1299,11 +1299,14 @@ the form ``MULT_i``, where *i* represents the constraint with which
 the multiplier is associated (counted from the order of declaration in
 the model block).
 
-The last type of auxiliary variables is introduced by the
+Auxiliary variables are also introduced by the
 ``differentiate_forward_vars`` option of the model block. The new
 variables take the form ``AUX_DIFF_FWRD_i``, and are equal to
 ``x-x(-1)`` for some endogenous variable ``x``.
 
+Finally, auxiliary variables will arise in the context of employing the 
+``diff``-operator.
+
 Once created, all auxiliary variables are included in the set of
 endogenous variables. The output of decision rules (see below) is such
 that auxiliary variable names are replaced by the original variables
@@ -1314,7 +1317,7 @@ variables is stored in ``M_.orig_endo_nbr``, and the number of
 endogenous variables after the creation of auxiliary variables is
 stored in ``M_.endo_nbr``.
 
-See `<https://archives.dynare.org/DynareWiki/AuxiliaryVariables>`__ for more technical details on auxiliary variables.
+See `<https://git.dynare.org/Dynare/dynare/-/wikis/Auxiliary-variables>`__ for more technical details on auxiliary variables.
 
 
 .. _init-term-cond: