Add comment and export Proccessor class

Author: Rafał Hibner

cpp/src/arrow/acero/accumulation_queue.h

@@ -127,7 +127,7 @@ class ARROW_ACERO_EXPORT SequencingQueue {
 class ARROW_ACERO_EXPORT SerialSequencingQueue {
  public:
   /// Strategy that describes how to handle items
-  class Processor {
+  class ARROW_ACERO_EXPORT Processor {
    public:
     virtual ~Processor() = default;
     /// Process the batch
@@ -142,10 +142,15 @@ class ARROW_ACERO_EXPORT SerialSequencingQueue {
     ///       be pretty fast and so are unlikely to block.
     virtual Status Process(ExecBatch batch) = 0;
 
+    /// Wrapper for processor with backpressure
+    ///
+    /// This wrapper adds backpressure logic acting on number of sequenced batches.
+    //  Also batches are Processes on new scheduled tasks. The tasks will be scheduled on
+    /// IO executor when requires_io==true.
     static std::unique_ptr<Processor> MakeBackpressureWrapper(Processor* processor,
                                                               BackpressureHandler handler,
                                                               ExecPlan* plan,
-                                                              bool requires_io = true);
+                                                              bool requires_io = false);
   };
 
   virtual ~SerialSequencingQueue() = default;