FirestoreSwift: Reword DocumentID breaking change in changelog (#10305)
FirestoreSwift: Reword DocumentID breaking change in changelog (#10305)

Updated the FirestoreSwift CHANGELOG to match the current state of the repo. Most users will be unaffected since the `@DocumentID` property wrapper constructors remain public but the changelog now warns that the `init(from documentReference: DocumentReference?)` constructor for `DocumentID` is internal.

In greater detail, the following is no longer possible:
```
let userDoc: DocumentReference = Firestore.firestore().document("users/foo")
do {
  let docID = try DocumentID<DocumentReference>(from: userDoc)
  if let docRef = docID.wrappedValue {
    print("Document ID: \(docRef.documentID)")
  }
} catch {
  print("DocumentID Error: \(error)")
}
```

The visibility change doesn't affect usage of `@DocumentID` though. The following is still possible (though it will trigger our warnings at runtime):
```
struct User: Codable {
  @DocumentID var docRef: DocumentReference? = Firestore.firestore().document("users/foo")
}
```
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