chore: document new boolean arg

compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/acir_variable.rs

@@ -1332,6 +1332,9 @@ impl AcirContext {
 
         // Optimistically try executing the brillig now, if we can complete execution they just return the results.
         // This is a temporary measure pending SSA optimizations being applied to Brillig which would remove constant-input opcodes (See #2066)
+        //
+        // We do _not_ want to do this in the situation where the `main` function is unconstrained, as if execution succeeds
+        // the entire program will be replaced with witness constraints to its outputs.
         if attempt_execution {
             if let Some(brillig_outputs) =
                 self.execute_brillig(&generated_brillig.byte_code, &b_inputs, &outputs)

compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs

@@ -266,6 +266,8 @@ impl Context {
 
         let code = self.gen_brillig_for(main_func, &brillig)?;
 
+        // We specifically do not attempt execution of the brillig code being generated as this can result in it being
+        // replaced with constraints on witnesses to the program outputs.
         let output_values = self.acir_context.brillig(
             self.current_side_effects_enabled_var,
             code,