Exclude help message is misleading

I think the `exclude` help message is a little bit confusing:

```
fdb> exclude
...
To find out whether it is safe to remove one or more of these
servers from the cluster, type `exclude '.
To return one of these servers to the cluster, type `include '.
```

The message could be interpreted that it's harmless to run `exclude ` but in fact this will exclude the addresses if those are not already excluded. I think we should add a warning above this command, that this will exclude those addresses if they are not already excluded.

Search the fdb CLI sources for the `exclude` help text shown in the issue. Add a warning explaining that `exclude ` performs the exclusion when the addresses are not already excluded, then verify the interactive help output.

Written by the indexing model from the issue text.