Explanation of submessage-reply flow in sylvia

[hashedone]

Addressing comment after prior review: #97 (review)

[jawoznia]

The sv::override_entry_points attribute can override the reply entry_point, but it is not needed as entry_points macro will generate it for you if the #[sv::msg(reply)] is present in the contract.

In the docs example we will present the sv::msg(reply) marked method with dispatch over the ID.