Add unit of measurement for download-buffer-size

I started getting these warnings `warning: download buffer is full; consider increasing the 'download-buffer-size' setting` but the documentation does not make it obvious what unit of measurement it accepts.

Author: kusold

src/libstore/filetransfer.hh

@@ -50,8 +50,9 @@ struct FileTransferSettings : Config
 
     Setting<size_t> downloadBufferSize{this, 64 * 1024 * 1024, "download-buffer-size",
         R"(
-          The size of Nix's internal download buffer during `curl` transfers. If data is
+          The size of Nix's internal download buffer in bytes during `curl` transfers. If data is
           not processed quickly enough to exceed the size of this buffer, downloads may stall.
+          The default is 67108864 (64MB).
         )"};
 };