StringReader.Read(Span<Char>) does not actually throw IOException

Hi,

The doc for `StreamReader.Read(Span)` claims that it'll throw an IOException if ["the number of characters read from the stream is larger than the buffer length."](
https://learn.microsoft.com/en-us/dotnet/api/system.io.streamreader.read?view=net-8.0#system-io-streamreader-read(system-span((system-char))))

This is not true, as seen with this code:
```csharp
var array = new char[8];

Span span = array;

File.WriteAllText("text.txt", "0123456789");

using StreamReader reader = new("text.txt");

var result = reader.Read(span);

Console.WriteLine(result);
Console.WriteLine(span.ToString());

// 8
// 01234567
```

[StringReader](https://learn.microsoft.com/en-us/dotnet/api/system.io.stringreader.read?view=net-8.0#system-io-stringreader-read(system-span((system-char)))) has the same behavior and is documented correctly:

```csharp

// StringReader
var array = new char[8];
var span = array;

using StringReader textReader = new("0123456789");

var result = textReader.Read(span);

Console.WriteLine(result);
Console.WriteLine(span.ToString());

// 8
// 01234567
```

If it's welcome, I would be happy to help fix the documentation