Comment about 'batch' option.

lib/zxystream.js

@@ -6,6 +6,11 @@ util.inherits(ZXYStream, stream.Readable);
 
 // Readable stream of line-delimited z/x/y coordinates
 // contained within the MBTiles `tiles` table/view.
+//
+// The `batch` option exists to allow tests to check that
+// multiple calls to `_read` are handled properly. IRL the
+// default offset of 10000 should be reasonably efficient
+// and not worth messing with.
 function ZXYStream(source, options) {
     if (!source) throw new TypeError('MBTiles source required');