Documentation for `only_for_reauth` is wrong

This issue has been migrated from [#14686](https://github.com/matrix-org/synapse/issues/14686).

---

### Description

The [documentation](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#password_config) for `password_config.enabled` says:

> Set to only_for_reauth to allow users with existing passwords to use them to log in and reauthenticate, whilst preventing new users from setting passwords.

In reality it seems that this settings only allows reauthentication, not log in.

The documentation should be fixed, but in addition **the feature that is advertised in the documentation should also be made available**, for example with a separate mode `password_config.enabled = "hidden"`, that allows users with existing passwords to use them to log in. The use case is system accounts (e.g. admin, bots) which don't exist in SSO (for example in Nextcloud, when enabling SSO and disabling passwords, [you can add `?direct=1` to the URL](https://github.com/nextcloud/user_oidc#disable-other-login-methods) to use username+password anyway).

### Steps to reproduce

- Set `password_config.enabled = "only_for_reauth"`
- Notice that Synapse doesn't advertise `m.login.password` as an authentication flow
- Try to log in with a username and password (you can't do this from unpatched Element), you will get `M_UNKNOWN: MatrixError: [400] Password login has been disabled`

### Homeserver

my own

### Synapse Version

1.73.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

PostgreSQL

### Workers

Single process

### Platform

Kubernetes

### Configuration

`password_config.enabled = "only_for_reauth"`

### Relevant log output

```shell
{"errcode":"M_UNKNOWN","error":"Password login has been disabled."}
```
```

### Anything else that would be useful to know?

_No response_