Update clump.rs docstring

[cefect]

grammar fixes

[cefect]

Also, it would be nice to add some clarity around the 'zero_back' option. From my tests, it seems like the tool still assigns a 'clump value' to the zero cells... but this is always category zero? (ie the 'zero_back' option propagates a zero=nodata assumption into the result). It seems like the tool should automatically set the nodata value to zero in this case, rather than rely on the user to ignore the category zero result cells.