Update docs for generated interop code

[Sergio0694]

This pull request updates the documentation for CsWinRT generic interface marshalling, refines the design for runtime callable wrappers (RCWs) and COM callable wrappers (CCWs), and clarifies the separation of type-specific and shared logic for generic interface marshalling. The changes introduce new marshaller interfaces and attributes, update method signatures for improved clarity and correctness, and shift the implementation strategy to use shared runtime adapter types rather than generating per-type adapters.

RCW marshalling and generic interface design

• Refactored the RCW base class to WindowsRuntimeEnumerator<T, TIIteratorMethods>, which is now generic over both the element type and a type-specific static interface for marshalling the Current method. The base class handles all IEnumerator<T> state management and semantics adaptation.
• Split the "Methods" logic into a static abstract interface IIteratorMethodsImpl<T> for type-specific marshalling (only Current), and a shared static class IIteratorMethods for non-type-specific methods (HasCurrent, MoveNext).
• Updated the generated RCW implementation (IEnumerator_stringNativeObject) to use the new generic base class and rely on the shared runtime logic, removing the need to override abstract methods.

Marshaller interfaces and attributes

• Renamed and updated marshaller interfaces and attributes to clarify their roles: IWindowsRuntimeUnsealedComWrappersCallback → IWindowsRuntimeUnsealedObjectComWrappersCallback, and updated method signatures to return CreatedWrapperFlags for additional wrapper metadata. [1] [2] [3]
• Updated marshaller attribute and callback implementations to use the new method signatures and fast-path logic, ensuring correct pointer handling and interface resolution. [1] [2]

CCW marshalling and runtime adapter strategy

• Shifted the CCW adapter implementation to use a generic runtime type IEnumeratorAdapter<T> defined in WinRT.Runtime.dll, replacing the previous strategy of generating per-type adapters and tables. This adapter provides correct semantics adaptation and efficient instance management via a shared ConditionalWeakTable.
• Updated documentation and example code to reflect the use of runtime-provided adapters and extension methods for type-specific operations, improving maintainability and reducing code generation complexity.

Vtable and method signature corrections

• Corrected the vtable method signature for get_Current to use HSTRING* for string marshalling and updated the implementation to use the shared runtime adapter and marshaller.

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These changes clarify the separation of type-specific and shared logic, improve marshalling correctness, and simplify the code generation and runtime infrastructure for generic interface support in CsWinRT.