Enumerable.Min<TSource>() documentation should mention null filtering behavior

In regards to `null`, the documentation states this:

> If `TSource` is a reference type and the source sequence is empty or contains only values that are `null`, this method returns `null`.

It also states:

> If type `TSource` implements [IComparable](https://docs.microsoft.com/en-us/dotnet/api/system.icomparable-1?view=net-6.0), the [Max(IEnumerable)](https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.max?view=net-6.0#system-linq-enumerable-max-1(system-collections-generic-ienumerable((-0)))) method uses that implementation to compare values. Otherwise, if type `TSource` implements [IComparable](https://docs.microsoft.com/en-us/dotnet/api/system.icomparable?view=net-6.0), that implementation is used to compare values.

This leads me to believe that the minimum value will be calculated purely based on the output of `IComparable.Compare(T, T)`. That is, the output of `.Min()` would be the same as `.OrderBy(x => x).First()` and the output of `.Max()` would be the same as `.OrderBy(x => x).Last()`. Or that if `Comparer.Default.Compare(a, b) < 0`, then the output of `new[] { a, b, }.Min()` would be `a`. However, I found the following behavior instead:

```csharp
Console.WriteLine($"Comparer.Default.Compare(null, \"a\") == {Comparer.Default.Compare(null, "a")}");
Console.WriteLine($"new[]{{ null, \"a\", }}.OrderBy(x => x).First() == {new[] { null, "a", }.OrderBy(x => x).First()}");
Console.WriteLine($"new[]{{ null, \"a\", }}.Min() == {new[] { null, "a", }.Min()}");
```

with output:

```
Comparer.Default.Compare(null, "a") == -1
new[]{ null, "a", }.OrderBy(x => x).First() ==
new[]{ null, "a", }.Min() == a
```

By experiencing this and examining [the](https://github.com/dotnet/runtime/blob/main/src/libraries/System.Linq/src/System/Linq/Min.cs#L614) [source](https://github.com/dotnet/runtime/blob/e3ecc8372630f22011815d64099598e30bcb43a7/src/libraries/System.Linq/src/System/Linq/Min.cs#L614), I know that `.Min()` and `.Max()` implicitly filter out any values equal to `null` as the first step (including nullable value types which box to `null`). However, this is not obvious from the documentation.

May you please alter the documentation to document this behavior?

Thanks!