Clarify procedure that finds layout

Explain how layout is generated, following https://github.com/mne-tools/mne-python/issues/994

I realized that part of the confusion stems from the fact that there are two different docstrings for the method in Evoked and for the top-level function in viz.py, and the two can get out of sync (the latter is probably updated more often). One way to fix this would be:

```
Evoked.plot_topomap.__func__.__doc__ = viz.plot_evoked_topomap.__doc__
```

tbh I'm not sure what viz.plot_evoked_topomap is useful for but that's a separate issue ;)