Clarify documentation - fill parameter only used for numerical data

[gtmaskall]

Discussed in #151

^{Originally posted by gtmaskall December 12, 2023}
The default behaviour of make_geocube with categorical data is to append a nodata string to the <varname>_categories variable. This makes sense from a Python indexing perspective inasmuch as the corresponding integer code for this is -1. This seems to override the fill argument. Again this makes sense with the use of -1 and the position of nodata. But if I'm correct in this, the documentation could be tightened up a little to clarify that fill only applies to numerical data.