LALSimulation: Update documentation in IMRNRWaveforms

During the review of the NR surrogates we noticed that the documentation points to an out of date document.

https://git.ligo.org/lscsoft/lalsuite/blob/master/lalsimulation/src/LALSimIMRNRWaveforms.c#L859

I believe the most up to date version of the documentation is https://arxiv.org/abs/1703.01076 with the corresponding DCC entry here https://dcc.ligo.org/LIGO-T1500606 However, one should also double check that the DCC and the arxiv are up to date.

Tagging @harald-pfeiffer to keep track of this