`az role assignment list` gives conflicting instructions about `--all` and `--scope`

## Describe the bug

**Command Name**
`az role assignment list`

**Errors:**
```
ValidationError: Parameter 'scope' can not be None.
```

## To Reproduce:

```
$ az role assignment list --role 'Network Contributor' --all
ValidationError: Parameter 'scope' can not be None.
$ az role assignment list --role 'Network Contributor' --all --scope /subscriptions/$SUBSCRIPTION
group or scope are not required when --all is used
```

## Expected Behavior

From the help text (`Show all assignments under the current subscription.`), I would expect `--all` not to produce an error, but instead to produce the same results as the following command:

```
$ az role assignment list --role 'Network Contributor' --scope /subscriptions/$SUBSCRIPTION
... (produces expected results)
```

## Environment Summary
```
Linux-5.4.0-58-generic-x86_64-with-debian-buster-sid
Python 3.6.10
Installer: DEB

azure-cli 2.18.0 *

```