Document what sort of intervals contig.add_dfe expects

[nspope]

In the docs https://popsim-consortium.github.io/stdpopsim-docs/stable/api.html#stdpopsim.Contig.add_dfe I think we should make it extra clear what sort of interval is expected; e.g. 0-based coordinate system and inclusive endpoint (I think that's the case based on the current text but am not entirely sure?)