Improve comments on options.writable_file_max_buffer_size (#9131)

Summary:
Comments of options.writable_file_max_buffer_size mentioned Windows, which is confusing. Remove it.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9131

Reviewed By: anand1976

Differential Revision: D32187003

fbshipit-source-id: 1f134d7ecdc4a9d13825d461ab1da56769b9455f

include/rocksdb/options.h

@@ -934,7 +934,7 @@ struct DBOptions {
   size_t random_access_max_buffer_size = 1024 * 1024;
 
   // This is the maximum buffer size that is used by WritableFileWriter.
-  // On Windows, we need to maintain an aligned buffer for writes.
+  // With direct IO, we need to maintain an aligned buffer for writes.
   // We allow the buffer to grow until it's size hits the limit in buffered
   // IO and fix the buffer size when using direct IO to ensure alignment of
   // write requests if the logical sector size is unusual