Updates for collection expressions clarity

[BillWagner]

Fixes #39586

This issue lists several elements. Here are the updates for each of them:

1. Add the list of potential interface types that a collection can be converted into. In addition, add an alert that in all cases, the collection is materialzed.
2. I didn't add a lot here. I think the addition about instantiating the collection is enough.
3. The additional list of interfaces as conversion targets has been added. I didn't specify the type generated, because that's an implementation detail that may not be visible, and may change.
4. I added a definition of a Countable type in the member access operator article. I linked between both. I don't think more is needed, as this fact is also emphasized in the definition of a well behaved collection.
5. Add the definition of a well-behaved collection.

Finally, do a general edit pass.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/operators/collection-expressions.md	Collection expressions - C# language reference
docs/csharp/language-reference/operators/member-access-operators.md	Member access operators and expressions - the dot, indexer, and invocation operators.