Add ArgumentNullException docs to JsonSerializer.SerializeAsync overloads

[alanisaac]

From dotnet/runtime#32705 (comment)

In System.Text.Json, the methods:

JsonSerializer.SerializeAsync<TValue>(Stream utf8Json, TValue value, JsonSerializerOptions? options = null, CancellationToken cancellationToken = default)
JsonSerializer.SerializeAsync(Stream utf8Json, object? value, Type inputType, JsonSerializerOptions? options = null, CancellationToken cancellationToken = default)

Both throw an ArgumentNullException when utf8Json is null, and when inputType is null for the latter method. Update the docs to reflect this.