Fix doc for WriteBatch::delete (#376)

The function does nothing if the key is not found in DB. See
1dd7873e08/include/rocksdb/write_batch.h (L85)

Fixes https://github.com/rust-rocksdb/rust-rocksdb/issues/352

src/db.rs

@@ -1765,9 +1765,7 @@ impl WriteBatch {
         }
     }
 
-    /// Remove the database entry for key.
-    ///
-    /// Returns an error if the key was not found.
+    /// Removes the database entry for key. Does nothing if the key was not found.
     pub fn delete<K: AsRef<[u8]>>(&mut self, key: K) -> Result<(), Error> {
         let key = key.as_ref();