Misleading documentation for func StringToStringVar

Quoting from the documentation of `func StringToStringVar`:
> The value of each argument will not try to be separated by comma.

However with the following program:
```go
res := make(map[string]string)
flag.StringToStringVar(&res, "foo-flag", res, "foo-flag-usage")
flag.Parse()
```
`go run ... --foo-flag "foo=bar,baz=quux"` results in `res = map[baz:quux foo:bar]`. 