Add documentation for metric parameters

fixes https://github.com/floweisshardt/atf/issues/119

see e.g. https://github.com/floweisshardt/atf/pull/140
- add description
- list default values
- list example configs

- [ ] `groundtruth` and `groundtruth_epsilon`
- [ ] `mode` SNAP/SPAN
- [ ] `series_mode`
- [ ] `min_window_size`