Update Exim configuration instructions.

In my `/etc/exim4/conf.d/main/02_exim4-config_options` I have:

```conf
untrusted_set_sender = *
```

The trusted users list is a bit confusing looking too:

```conf
# trusted users can set envelope-from to arbitrary values
.ifndef MAIN_TRUSTED_USERS
MAIN_TRUSTED_USERS = uucp
.endif
trusted_users = MAIN_TRUSTED_USERS
.ifdef MAIN_TRUSTED_GROUPS
trusted_groups = MAIN_TRUSTED_GROUPS
.endif
```