change docstring in tracers.py/translate_IA_norm

[nikosarcevic]

I think we should be more clear about the last two arguments in translate_IA_norm method in tracers.py.

I think it is important to emphasize that Om_m_fid should be given a fiducial value (different from the default 0.3) when setting Om_m2_for_c2 to True.
The current explanation we have now is a bit confusing regarding this.

[nikosarcevic]

I can propose the different wording if you want. Let me know how to proceed.