Strings as list don't work, maybe clarify the documentation

[jackschmidt]

In gap-system/gap#4585 @mfarrokhidg asked about using Strings as list of characters as arguments. This does not work. The documentation tries to address this ("arguments must be GAP Strings"), but perhaps the documentation should be more clear that the functions require IsStringRep not just IsString.

I gave the following workaround. A similar example might be useful in the manual.

gap> x:=['t', 'e', 's', 't'];
"test"
gap> IO_mkdir(x,0);
fail
gap> ConvertToStringRep(x);
gap> IO_mkdir(x,0);
true