Merge concurrent/sequential overloads are not at all obvious

Currently the overload `Merge(IAsyncEnumerable[])` is concurrent while `Merge(IEnumerable>)` and `Merge(IAsyncEnumerable>)` are sequential, this is not at all intuitive without looking at the source code.

I'd argue `Merge` should always be a concurrent operator as merge implies items can be interleaved, `Concat` should be the sequential version of this.

You can see [a comment](https://github.com/dotnet/reactive/blob/c940949fe61fe984ad42a71d3ddff5d40540dc92/Ix.NET/Source/System.Interactive.Async/System/Linq/Operators/Merge.cs#L319-L335) in the source code explaining this complexity.

It looks like the breaking change for previous users the sequential Merge should have been made with the release of the BCL interfaces but was not done so.