Update alias help.

[fnando]

What

The original --alias implementation had --force to overwrite existing contract id aliases, but @leighmcculloch suggested that it'd be better to always use the latest id. Now, @willemneal created a ticket to add that back. Until we come to a decision, let's make the flag help explicit.

Why

So people can know that --alias will always store the latest contract id without asking for confirmation.

Known limitations

N/A

[willemneal]

Yeah to me we should make the redeploy easier, but devs are currently required to include a redeploy method. But once we have the snapshot stuff we can copy over the state to the newly deployed contract and all should be okay.