6-arg version of fs.write doesn't support string as the 2nd arg,
and similarly 5-arg version fs.writeSync doesn't support string
as the 2nd arg. This change removes those unsupported types from
the API docs.

(string type is supported for 5-arg version of fs.write, and 4-arg version of fs.writeSync and those are covered by the next entries)

Examples to support the above claim:

fs.write(process.stdout.fd, Buffer.from([0x61]), 0, 1, null, () => {});
// => prints 'a'
fs.write(process.stdout.fd, "a", 0, 1, null, () => {});
// => throws ERR_INVALID_CALLBACK