I think we need to document the `old_value` param in the list of params below, and then this line of documentation should probably be updated to read something like:

```
It returns GIT_EMODIFIED if the reference's value at the time of update
does not match the 'old_value' parameter (i.e. if the reference has been
moved by another process or thread from the expected previous value).
```

My wording isn't great, so feel free to substitute your own, but the sentence that is here feels a little ambiguous to me.