Document `fillarray.f` (fillarray with string arg that reads from a file)

[eleses]

I see (in the code) there's an undocumented variant of fillarray that takes a string as argument and reads an array from there at i-time into an i- or k-array.

The related opcode string2array intro'd in 6.13 (according to the change log) also seems to be lacking a manual page. I'm guessing fillarray with string arg was also introduced in the same Csound version?

Also, I had discovered a while back one can fill string arrays too (that's fillarray.s), although this is also not mentioned in the manual.