Add more info to WCS reference files documentation

[rizeladiaz]

1. Add a little more info to the description of the CAMERA reference file. Something like "This file contains the coordinate transforms from the NIRSpec focal plane array to the grating wheel assembly."
2. Add a little more clarification to the description of the COLLIMATOR reference file. Something like "This file contains the coordinate transforms from the NIRSpec grating wheel assembly to the microshutter array (MSA)."
3. Add a little more description to the FORE reference file. Something like "includes the Wavelength-dependent coordinate transforms from the NIRSpec microshutter array through the filter to the instrument pickoff mirror."
4. Add more clarification to the description of the OTE file; e.g., "Coordinate transforms from the NIRSpec filter through the telescope to the sky."