message: Added more info to fast fields.
diff: @@ -130,7 +130,16 @@ impl SchemaBuilder {
  self.add_field(field_entry)
  }
  - /// Adds a fast bytes field to the schema
  + /// Adds a fast bytes field to the schema.
  + ///
  + /// Bytes field are not searchable and are only used
  + /// as fast field, to associate any kind of payload
  + /// to a document.
  + ///
  + /// For instance, learning-to-rank often requires to access
  + /// some document features at scoring time.
  + /// These can be serializing and stored as a bytes field to
  + /// get access rapidly when scoring each document.
  pub fn add_bytes_field(&mut self, field_name: &str) -> Field {
  let field_entry = FieldEntry::new_bytes(field_name.to_string());
  self.add_field(field_entry)
  