Fix docs about rate limit overrides

### Description

Synapse allows to override rate limits per-user: https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html#override-ratelimiting-for-users

The following is unclear:
- "`messages_per_second` - integer - The number of actions that can be performed in a second. 0 mean that ratelimiting is disabled for this user."
- this seems to actually only mean messages, not actions. it appears to not be possible to override rc for other actions here.
- "[Delete ratelimit](https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html?highlight=rate%20limit%20override#delete-ratelimit)" ideally the subheadings would be clearer about the fact that one is adjusting rate limit *overrides*, not rate limits. I.e. "delete ratelimit" does not disable the ratelimit of a user, but sets it to the global default.

### Steps to reproduce

- try to read docs
- try to override rate limit to enable more invites

### Homeserver

another

### Synapse Version

1.136.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

psql via mdad

### Workers

Multiple workers

### Platform

mdad on debian

### Configuration

_No response_

### Relevant log output

```shell
n/a
```

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

_No response_