FstatisticTools.h: update documentation header after it was renamed a while ago

@karl-wette Looking for uses of PulsarAmplitudeParams, I got confused by the documentation apparently showing a code duplication between FStatisticTools.h and EstimateAmplitudeParams.h, until realizing that the latter is just a phantom generated by an outdated doxygen header. ;)

Also dropping here the instructions for how to #include, as it seems nonstandard compared to the rest of the library, but feel free to overrule.