Add option to disable masering workaround in documentation

[ThomasCeulemans]

In solver.tpp::correct_negative_opacities, we apply a somewhat hardhanded fix to the level populations in order to avoid population inversions (in order to avoid very low/negative opacities and optical depths later on, which our numerical solver cannot handle. We do however offer some parameter to tweak this behavior: (see parameters.hpp::population_inversion_fraction). This value can be set to -1 (or any negative number) to skip this fix.

Evidently, I should add this info to the documentation.