Adding more documentation to ReadOptions.set_readahead_size [skip ci]...

src/db.rs

@@ -1722,6 +1722,11 @@ impl ReadOptions {
         unsafe { ffi::rocksdb_readoptions_set_total_order_seek(self.inner, v as c_uchar) }
     }
 
+    /// If non-zero, an iterator will create a new table reader which
+    /// performs reads of the given size. Using a large size (> 2MB) can
+    /// improve the performance of forward iteration on spinning disks.
+    /// Default: 0
+    ///
     /// ```
     /// use rocksdb::{ReadOptions};
     ///