Document that set_fd_flags can't set most flags

[sunfishcode]

Many OS's don't support changing most of the descriptor flags on an open descriptor. The two flags WASI has that do seem like they might be portable to allow setting are append and nonblock (though of course nonblock is not implemented for files in most situations anyway).

We should at least document this.

And perhaps we should even remove set-fd-flags and replace it with just set-append and set-nonblock which operate on individual flags.