Better diagnostics for nullness inference related errors

I think it would be good to be more explicit about nullness inference happening behind the scenes.

For the code
```
let xs = [| ""; ""; null |]
```
we'll get
> Nullness warning: The type 'string' does not support 'null'

It would be helpful to have something like

> "xs was inferred to have the type `string[]` but got null among its values. Either explicitly specify null in the return type (`(string | null)[]`), or remove null from the collection - or suppress the warning but no please don't."

_Originally posted by @psfinaki in https://github.com/dotnet/fsharp/pull/15181#discussion_r1679527189_