Diagnostic is unclear when passing ref readonly to ref readonly parameter using 'ref'

**Version Used**: fca6e1fcdcded85e69cc32e15acfb6820cd45597

**Steps to Reproduce**:

[SharpLab](https://sharplab.io/#v2:EYLgtghglgdgNAFxFANnAJiA1AHwAIBMABAMICwAUAN6VF1F4AsRAsgAwAUATgKYBmRXhHQB7GCgCeRWAmkBKIlSIBfWvSasAjN36CewsZOkxZUOWro0K9G6068BZgNxEA9K6I8uXEV1IBlAA4AZgIAThBSCBgYEVkAVwBnHiIANwguKAhgFBSAcig8oghE4r0BXyIReNl0lHiU4B4AYwgklKhTUog9A3EpdMzs3Itbdg5YeSdR1QplIA===)

```cs
class C
{
void M0(ref readonly int i) { }
void M1(ref readonly int i)
{
M0(ref i); // error CS8329: Cannot use variable 'i' as a ref or out value because it is a readonly variable
M0(in i); // ok
}
}
```

It appears that this error is [by design](https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/ref-readonly-parameters.md#value-kind-checks). @jjonescz are there notes handy for why the design is this way?

Given the above I think the diagnostic message should simply be adjusted here to indicate that the user should use `in` at the call side instead.

I personally found it surprising that `ref` was not permitted here, because other kinds of arguments can be passed using `ref` for this parameter without issue. It was not obvious to me that using `ref` was actually requiring the argument to be a writable reference, even though the parameter will not assign to the referent.