az keyvault role assignment create documentation mismatch

### Describe the bug

The documentation for param `--assignee-principal-type` states that:

```
accepted values: Application, DirectoryObjectOrGroup, DirectoryRoleTemplate, Everyone, ForeignGroup, Group, MSI, ServicePrincipal, Unknown, User
```

However, running with `--assignee-principal-type MSI` yields:

`ERROR: az role assignment create: 'MSI' is not a valid value for '--assignee-principal-type'. Allowed values: User, Group, ServicePrincipal, ForeignGroup.` during pipeline run.

Running with:

```
az role assignment create --assignee-object-id $CLUSTER_MSI_PRINCIPAL_ID \
--role "Key Vault Crypto Service Encryption User" \
--assignee-principal-type ServicePrincipal \
--scope $CLUSTER_KV_ID
```

Where the `$CLUSTER_MSI_PRINCIPAL_ID` is fetched through:

```
CLUSTER_MSI_PRINCIPAL_ID=$(az identity show --name $CLUSTER_MSI \
--resource-group $CLUSTER_RG \
--query 'principalId' \
--output tsv)
```

as suggested in https://github.com/Azure/azure-cli/issues/11594#issuecomment-1047918766 worked.

### Related command

az role assignment create

### Errors

ERROR: az role assignment create: 'MSI' is not a valid value for '--assignee-principal-type'. Allowed values: User, Group, ServicePrincipal, ForeignGroup.

### Issue script & Debug output

CLUSTER_MSI_PRINCIPAL_ID=$(az identity show --name $CLUSTER_MSI \
--resource-group $CLUSTER_RG \
--query 'principalId' \
--output tsv)

az role assignment create --assignee-object-id $CLUSTER_MSI_PRINCIPAL_ID \
--role "Key Vault Crypto Service Encryption User" \
--assignee-principal-type MSI \
--scope $CLUSTER_KV_ID

### Expected behavior

Consistency between CLI and documentation

### Environment Summary

/usr/bin/az --version
azure-cli 2.53.0

core 2.53.0
telemetry 1.1.0

Extensions:
azure-devops 0.26.0

Dependencies:
msal 1.24.0b2
azure-mgmt-resource 23.1.0b2

### Additional context

_No response_