Update FSharp.Core API reference to use array instead of []

[edgarfgp]

Now that #13700 is merged and based on Don's comment here fsharp/fslang-suggestions#635 (comment). Seems like the pending work is:

• Update FSharp.Core API reference
• Update F# docs at docs.microsoft.com/dotnet/fsharp
• Update the F# style guide at docs.microsoft.com/dotnet/fsharp