Document why disjoint union doesn't work with .

theories/algebra/boolset.v

@@ -2,7 +2,9 @@ From stdpp Require Export sets boolset.
 From iris.algebra Require Export cmra sets.
 Set Default Proof Using "Type".
 
-(** Union CMRA for [boolset] (see [sets]). *)
+(** Union CMRA for [boolset] (see [sets]). It doesn't seem possible,
+without axioms, to show [##] decidable on [boolset], which is required
+by the disjoint union CMRA's operation (see [set_disj_op]). *)
 
 Section boolset.
   Context `{EqDecision A}.