Clarify method hiding explanation in polymorphism documentation

[Copilot]

The polymorphism documentation's explanation of type casting with hidden base class members was confusing readers. The original text didn't clearly distinguish between method hiding (using new) and method overriding (using override), leading to confusion about why casting affects which method is called.

Changes Made

• Clarified method hiding vs overriding: Added explanation that new keyword creates method hiding rather than overriding
• Explained compile-time vs runtime behavior: Clarified that with method hiding, the method called depends on the compile-time type of the variable, not the runtime type of the object
• Added concrete example explanation: Provided clear explanation of why the same object instance calls different methods when accessed through different variable types

Before

Hidden base class members may be accessed from client code by casting the instance of the derived class to an instance of the base class. For example:

After

When you use the `new` keyword, you're creating a method that *hides* the base class method rather than *overriding* it. This is different from virtual methods. With method hiding, the method that gets called depends on the compile-time type of the variable, not the runtime type of the object.

Hidden base class members can be accessed from client code by casting the instance of the derived class to an instance of the base class. For example:

:::code language="csharp" source="./snippets/inheritance/Inheritance.cs" ID="SnippetUseNewMethods":::

In this example, both variables refer to the same object instance, but the method that gets called depends on the variable's declared type: `DerivedClass.DoWork()` when accessed through the `DerivedClass` variable, and `BaseClass.DoWork()` when accessed through the `BaseClass` variable.

This addresses the confusion expressed in the original issue where the user asked: "if i will type cast derived class object as base class then it will call base class method, but without casting it calling to the base method then what's meaning of type casting?"

Fixes #27506.

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docs/csharp/fundamentals/object-oriented/polymorphism.md	docs/csharp/fundamentals/object-oriented/polymorphism

[BillWagner]

This LGTM. I'll mark it ready for final review.