[Documentation] Update get account docs

[sideninja]

Here it says you can only get auth account in signed transactions:
https://docs.onflow.org/cadence/language/accounts/#authaccount
I believe this has now changed and you can do that in scripts as well. The docs should be updated?

[SupunS]

Probably it has meant to say that, for transactions, only signed transactions can get the auth account. i.e: each signer is passed as an argument to the prepare block.

Maybe we could reword it.

In the next paragraph, it says scripts can also get

A script can get the AuthAccount for an account address using the built-in getAuthAccount function: