workspacedir doesn't actually support relative paths?

[Documentation](https://docs.buildstream.build/master/using_config.html) states that `workspacedir` can be a relative path:

> Because of this, the workspacedir directory is the only directory which is allowed to be declared as a relative path.

However setting `workspacedir: ./workspace/` in `~/.config/buildstream.conf` and then executing `bst workspace list` produces
```
$ bst workspace list
Error loading user configuration: workspacedir must be an absolute path
```

Reproduce the issue with `workspacedir: ./workspace/` in `~/.config/buildstream.conf` and run `bst workspace list`. Compare the observed absolute-path validation with the behavior documented at the linked configuration page, then determine whether the implementation or documentation is wrong. Done means the documented behavior and the command's result agree, with coverage for the relative-path case if the project has a suitable test location.

Written by the indexing model from the issue text.