docs: update info about varint native data type

Author: muzarski

docs/source/data-types/data-types.md

@@ -27,7 +27,7 @@ Database types and their Rust equivalents:
 * `Timestamp` <----> `value::CqlTimestamp`, `chrono::DateTime<Utc>`, `time::OffsetDateTime`
 * `Duration` <----> `value::CqlDuration`
 * `Decimal` <----> `bigdecimal::Decimal`
-* `Varint` <----> `num_bigint::BigInt`
+* `Varint` <----> `value::CqlVarint`, `num_bigint::BigInt` (v0.3 and v0.4)
 * `List` <----> `Vec<T>`
 * `Set` <----> `Vec<T>`
 * `Map` <----> `std::collections::HashMap<K, V>`

docs/source/data-types/varint.md

@@ -1,5 +1,16 @@
 # Varint
-`Varint` is represented as [`num_bigint::BigInt`](https://docs.rs/num-bigint/0.4.0/num_bigint/struct.BigInt.html)
+`Varint` is represented as `value::CqlVarint` or [`num_bigint::BigInt`](https://docs.rs/num-bigint/0.4.0/num_bigint/struct.BigInt.html).
+
+## num_bigint::BigInt
+
+To make use of `num_bigint::BigInt` type, user should enable one of the available feature flags (`num-bigint-03` or `num-bigint-04`). They enable support for `num_bigint::BigInt` v0.3 and v0.4 accordingly.
+
+## value::CqlVarint
+
+Without any feature flags, the user can interact with `Varint` type by making use of `value::CqlVarint` which
+is a very simple wrapper representing the value as signed binary number in big-endian order.
+
+## Example
 
 ```rust
 # extern crate scylla;