Add doc for `set_cache_index_and_filter_blocks` (#699)

src/db_options.rs

@@ -635,6 +635,9 @@ impl BlockBasedOptions {
         }
     }
 
+    /// If cache_index_and_filter_blocks is enabled, cache index and filter blocks with high priority.
+    /// If set to true, depending on implementation of block cache,
+    /// index and filter blocks may be less likely to be evicted than data blocks.
     pub fn set_cache_index_and_filter_blocks(&mut self, v: bool) {
         unsafe {
             ffi::rocksdb_block_based_options_set_cache_index_and_filter_blocks(