GH-50394: [Docs][C++] Reduce Sphinx warnings when building HTML

cpp/apidoc/Doxyfile

@@ -2484,6 +2484,8 @@ PREDEFINED             = __attribute__(x)= \
                          ARROW_EXTERN_TEMPLATE= \
                          ARROW_FLIGHT_EXPORT= \
                          ARROW_FLIGHT_SQL_EXPORT= \
+                         ARROW_SUPPRESS_DEPRECATION_WARNING= \
+                         ARROW_UNSUPPRESS_DEPRECATION_WARNING= \
                          GANDIVA_EXPORT= \
                          PARQUET_EXPORT=
 

cpp/src/arrow/dataset/file_base.h

@@ -451,12 +451,14 @@ struct ARROW_DS_EXPORT FileSystemDatasetWriteOptions {
 
   /// Callback to be invoked against all FileWriters before
   /// they are finalized with FileWriter::Finish().
+  /// \hideinitializer
   std::function<Status(FileWriter*)> writer_pre_finish = [](FileWriter*) {
     return Status::OK();
   };
 
   /// Callback to be invoked against all FileWriters after they have
   /// called FileWriter::Finish().
+  /// \hideinitializer
   std::function<Status(FileWriter*)> writer_post_finish = [](FileWriter*) {
     return Status::OK();
   };

cpp/src/arrow/dataset/scanner.h

@@ -150,6 +150,7 @@ struct ARROW_DS_EXPORT ScanOptions {
   std::vector<FieldRef> MaterializedFields() const;
 
   /// Parameters which control when the plan should pause for a slow consumer
+  /// \hideinitializer
   acero::BackpressureOptions backpressure =
       acero::BackpressureOptions::DefaultBackpressure();
 };