Mention self-insertion UB in docs of `CollectionExtensions.InsertRange`

[karakasa]

Describe the issue or suggestion

https://github.com/dotnet/runtime/blob/102f28e5a64cdbe89484ac10414fdebb9a51c65f/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/CollectionExtensions.cs#L140-L141

Per code, inserting a span from a List<T> into itself is UB. I think we should mention it in the docs, because List<T>.InsertRange(itself) does work but the Span version doesn't.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-collections