Explain that contracts return excessive funds upon deployment with Deployable trait

[novusnota]

And that it's done automatically when the contract uses the Deployable trait.

Also, mention that receivers don't do that by default — in order to do refunds in receivers one has to use, say, self.notify("Cashback".asComment()); or alike by the end of their body.

P.S.: Without inheriting the Deployable trait, the contract can still handle the Deploy message like so:

import "@stdlib/deploy"; // to populate ctx and have Deploy msg in sight

contract Example {
    receive(msg: Deploy) {} // no-op, won't return the funds now
}

[anton-trunov]

Actually, refunding on deployment should not be done, this is a bug and should be fixed. The contract should manage its gas so that it has enough funds to pay for storage at all times. And a good practice is to introduce a "topup" receiver that can be used to top up the contract's balance in case there is still not enough gas to pay for storage.