modified too big message in evaluate_planner_objective

Mention the issue in the doc

doc/dynare.texi

@@ -7059,7 +7059,7 @@ The planner objective must be declared with the @code{planner_objective} command
 
 This command only creates the expanded model, it doesn't perform any
 computations. It needs to be followed by other instructions to actually
-perfrom desired computations. Note that it is the only way to perform
+perform desired computations. Note that it is the only way to perform
 perfect foresight simulation of the Ramsey policy problem.
 
 @xref{Auxiliary
@@ -7200,7 +7200,9 @@ In contrast, the second entry stores the value of the planner objective with
 initial Lagrange multipliers of the planner's problem set to 0, i.e. it is assumed 
 that the planner succumbs to the temptation to exploit the preset private expecatations 
 in the first period (but not in later periods due to commitment).
-
+Because it entails computing at least a second order approximation, this
+computation is skipped with a message when the model is too large (more than 180 state
+variables, including lagged Lagrange multipliers).
 @customhead{Steady state}
 @xref{Ramsey steady state}.
 

matlab/evaluate_planner_objective.m

@@ -34,7 +34,7 @@ nstatic = M.nstatic;
 nspred = M.nspred;
 if nspred > 180
     disp(' ')
-    disp(['evaluate_planner_objective: model too large, can''t evaluate planner ' ...
+    disp(['WARNING in evaluate_planner_objective: model too large, can''t evaluate planner ' ...
           'objective'])
     return NaN
 end