clean up lalsimulation.nrfits doxygen

@vijay.varma we forgot to look carefully at the final doxygen output of the new module. E.g. here I see a bunch of small issues in the eval_nrfit() documentation:

• Undefined control sequence \cross (well, at least this isn't crashing the build!) -> use \times, I guess
• missing line-breaks in the description of arguments (actually it would be nicer to move those to the function header itself, though I'm not sure if this is supported for python the same way as for C)
• "These spin components" block somehow got quoted/verbatimed or something.
• Missing (double) linebreak again at the end before "Example".

I haven't gone through the rest of the module yet, but can post more items later; or you have a first go through all of it and I double-check once you have a WIP:MR. (No need to go through the waveforms review repo; you can just make a quick branch on your own fork and submit a MR from there.)

cc @nathan-johnson-mcdaniel