add comment

include/rocksdb/c.h

@@ -601,6 +601,8 @@ extern void rocksdb_options_set_max_grandparent_overlap_factor(
 extern void rocksdb_options_set_max_bytes_for_level_multiplier_additional(
     rocksdb_options_t*, int* level_values, size_t num_levels);
 extern void rocksdb_options_enable_statistics(rocksdb_options_t*);
+
+/* returns a pointer to a malloc()-ed, null terminated string */
 extern char *rocksdb_options_statistics_get_string(rocksdb_options_t *opt);
 
 extern void rocksdb_options_set_max_write_buffer_number(rocksdb_options_t*, int);