message: Add doc comments to enum Evaluated
diff: @@ -8,6 +8,24 @@ use crate::result::Result;
  use super::EvaluateError;
  +/// `LiteralRef`, `Literal` and `StringRef` are used when it is not possible to specify what kind of `Value`
  +/// can be applied.
  +///
  +/// * `1 + 1` is converted into `LiteralRef + LiteralRef`, `LiteralRef` of `1` can
  +/// become `Value::I64` but it can be also `Value::F64`.
  +///
  +/// * Specifing column `id`, it is converted into `ValueRef` because `id` can be specified from table
  +/// schema.
  +///
  +/// * `"hello"` is converted into `StringRef`, similar with `LiteralRef` but it is for storing
  +/// string value.
  +///
  +/// * Evaluated result of `1 + 1` becomes `Literal`, not `LiteralRef` because executor has
  +/// ownership of `1 + 1`.
  +///
  +/// * Similar with `Literal`, `Value` is also generated by any other operation with `ValueRef` or
  +/// `Value`.
  +/// e.g. `LiteralRef` + `ValueRef`, `LiteralRef` * `Value`, ...
  pub enum Evaluated<'a> {
  LiteralRef(&'a AstValue),
  Literal(AstValue),
  