Add note about IsExternalInit

[RReverser]

Some of the newer C# features are supported by Unity compiler, but require symbols from newer .NET that Unity doesn't have.

One notable feature that we rely on a lot and that users run into is init-only properties - either implicit as part of record declarations, or explicit ones like MyType MyProp { get; init; }.

This compiles fine in Unity, but complains about missing System.Runtime.CompilerServices.IsExternalInit symbol. The issue is pretty googlable, is mentioned in the Unity docs and the required fix is just for the user to add

namespace System.Runtime.CompilerServices
{
    internal static class IsExternalInit { }
}

to their own project (the one with our generated C# files), but we need to add this to our C# docs for better visibility as well.

[cloutiertyler]

What does it mean that we "rely on it a lot"? I think this would have to rise to a pretty substantial amount of need to pay for the cost of confusing new users or causing frustration for users