Incorrect description of ImmutableSet.Builder<T>.UnionWith(IEnumerable<T>)

The [description](https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable.immutablesortedset-1.builder.unionwith?view=net-8.0#system-collections-immutable-immutablesortedset-1-builder-unionwith(system-collections-generic-ienumerable((-0)))) currently says that this function "Modifies the current set so that it contains all elements that are present in both the current set and in the specified collection."

That sure sounds to me like an intersection (or at best a no-op), not a union. Seems like this should be changed to be more in line with the [description from ```ISet```](https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.iset-1.unionwith?view=net-8.0#system-collections-generic-iset-1-unionwith(system-collections-generic-ienumerable((-0)))), which is "Modifies the current set so that it contains all elements that are present in the current set, in the specified collection, or in both."

I don't really know how automatically the documentation updates, so to be clear just in case, this incorrect description is also contained in the function's brief entry on [the documentation page for the class](https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable.immutablesortedset-1.builder?view=net-8.0).

It or something similar may or may not also be present in the documentation for similar functions in similar interfaces and/or classes; I have not checked any other than ```ImmutableSet.Builder``` itself.

Open the ImmutableSet.Builder.UnionWith(IEnumerable) API entry and compare its description with ISet.UnionWith. Also check the brief entry on the ImmutableSet.Builder class documentation page. Done means both references accurately describe union semantics and use consistent wording.

Scritto dal modello di indicizzazione a partire dal testo della issue.