correct default location, mention abs path

Default in _inject_zeo_conf() is '.../%(fs_part_name)s.fs', not .../Data.fs.

Absolute paths are fine too -- os.path.join() will discard previous path components when encountering an absolute path, so when location is an absolute path, `os.path.join(self.buildout['buildout']['directory'], location)` is an NOP.