docs(jans-cli-tui): clarify userId and username behavior

**Clarify `userId` and `username` behavior in Jans CLI and TUI**

There is an inconsistency in how the username is handled when creating a user through Jans TUI and Jans CLI.

**Observed behavior**

**Jans TUI**

When creating a user through Jans TUI, the **Username** field is required.

**Jans CLI**

Using the `post-user` operation, a user can be created without providing `userId`:

```bash
jans cli --operation-id post-user \
--data '{
"givenName": "Test",
"displayName": "Test User",
"mail": "user@example.com",
"userPassword": "your-password",
"status": "active"
}'
```

A user can also be created by providing `userId`. When `userId` is provided, its value is displayed as the user's username.

**Clarification needed**

Please clarify the following:

1. Is `userId` the same attribute as the **Username** shown in Jans TUI, or are they different attributes?
2. Is `Username/userId` required for user creation in Jans TUI, while it is optional when creating a user through Jans CLI?

**Documentation impact**

Clarifying this behavior will allow the Jans CLI and TUI documentation to accurately describe the user creation requirements and avoid confusion around `userId` and `userName`.