amend some comments in util-print.c

amend some comments in util-print.c

When I needed to print a string and didn't remember which of the three
functions fn_print(), fn_printn() and fn_printzp() was the right one
for the data, every time it would end up in reading through all of them
and forgetting the difference shortly after the commit.

Just having it explained in the comments should work better.