Iterate on unban docs?

The following quote is from the js-sdk, which feels like it might be useful knowledge to translate into the spec, whatever it means:
```
// unbanning != set their state to leave: this used to be
// the case, but was then changed so that leaving was always
// a revoking of privilege, otherwise two people racing to
// kick / ban someone could end up banning and then un-banning
// them.
```

Start by reading the quoted unban behavior in the js-sdk and comparing it with the relevant Matrix specification text. Clarify whether the distinction between unbanning, leaving, and revoking privilege is already specified; done means the spec documents the intended behavior unambiguously.

Written by the indexing model from the issue text.