Document `tai2` and `utc2` slightly more

[nandoconde]

It may not be necessary, but on a first cursory look the meaning of these arguments was not apparent to me.

Would it be good to add some documentation specifying that tai1 and utc1 expect integer Julian Day and tai2 and utc2 expect fraction of the day?

I took the meaning of these arguments from here: https://ascom-standards.org/Help/Developer/html/M_ASCOM_Astrometry_SOFA_SOFA_UtcTai.htm