NOBUG Adding documentation

src/query/similarity.rs

@@ -2,6 +2,7 @@ use Score;
 use query::Explanation;
 use query::MultiTermAccumulator;
 
+/// Similarity based scoring.
 pub trait Similarity: MultiTermAccumulator {
     fn score(&self, ) -> Score;
     fn explain(&self, vals: &[(usize, u32, u32)]) -> Explanation;