Document different behaviours of landmark mappings file better

[patrikhuber]

See this code section here: https://github.com/patrikhuber/eos/blob/9ac31018893bc1f3adda7382725c383e1560cfac/include/eos/fitting/fitting.hpp#L400-L420

I am not sure whether this is documented somewhere, I'd have to check. If it isn't already, would be worth documenting this more prominently.