Draft: Update documentation for XLALSimInspiralfLow2fStart function

Description

The documentation string for the function XLALSimInspiralfLow2fStart in lalsuite/lalsimulation/lib/LALSimInspiral.c here lacked detailed explanations for fStart, fLow, and ampOrder. This merge request aims to improve the documentation. No code changes have been made; only the documentation of the function mentioned above has been updated.