cabal v2-run requires a TARGET when passing flags

**Describe the bug**
The documentation states a TARGET only has to be specified if necessary to disambiguate. However, if you want to pass a flag to an executable it seems TARGET is non-optional.

**To Reproduce**
Steps to reproduce the behavior:

```
# In a project with a single executable which accepts the `-t` flag:
$ cabal v2-run -- -t
```

Please use version-prefixed commands (e.g. `v2-build` or `v1-build`) to avoid ambiguity.

**Expected behavior**
Since there's only a single possible target the documentation suggests it does not need to be specified.
Either the documentation or the functionality should be updated.