DOCUMENTATION: Changed source docstring for phase argument

Addresses Issue 649. The current description for the "phase" argument in source.py is misleading as "phase" is not usually the phase at coalescence; for most waveforms it is the phase at the reference frequency. See LALSiminspiral Docs. The "phase" input is phase at peak for waveforms such as IMRPhenomP and IMRPhenomPv2, but is the phase at f_ref for TaylorF2, IMRPhenomXPHM, etc.