Document authentication process when using Dictionary.

(you don't have to strictly follow this form)

**Describe the unexpected behaviour**
When trying to use ``getDict`` there is an authentication error thrown.

**How to reproduce**
ClickHouse server version 23.9.1 revision 54466.

Have a plaintext password setup for the default user.

Run the code:
```sql
create table test(a Int64, b String) Engine=MergeTree order by a;
insert into test values (1,'a'), (2,'b');

CREATE DICTIONARY test_dict( `a` Int64,`b` String)
PRIMARY KEY a
SOURCE(CLICKHOUSE(QUERY 'SELECT a,b FROM default.test'))
LIFETIME(MIN 0 MAX 0) LAYOUT(FLAT());

select dictGet('test_dict', 'b', 1);
```

**Expected behavior**
You get the result of ``dictGet``

**Error message and/or stacktrace**
```
Received exception from server (version 23.9.1):
Code: 516. DB::Exception: Received from localhost:9000. DB::Exception: default: Authentication failed: password is incorrect, or there is no user with such name.

If you have installed ClickHouse and forgot password you can reset it in the configuration file.
The password for default user is typically located at /etc/clickhouse-server/users.d/default-password.xml
and deleting this file will reset the password.
See also /etc/clickhouse-server/users.xml on the server where ClickHouse is installed.

: While processing dictGet('test_dict', 'b', 1). (AUTHENTICATION_FAILED)
```

**Additional context**
None