Clarify StartupObject documentation: when it's needed and qualification requirements (#47726)

* Initial plan

* Fix StartupObject documentation to clarify when it's needed and qualification requirements

Co-authored-by: BillWagner <[email protected]>

* Update docs/csharp/language-reference/compiler-options/advanced.md

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>

Author: Copilot

docs/csharp/language-reference/compiler-options/advanced.md

@@ -56,13 +56,13 @@ You add any of these options in a `<PropertyGroup>` element in your `*.csproj` f
 
 ## StartupObject
 
-This option specifies the class that contains the entry point to the program, if more than one class contains a `Main` method.
+This option specifies the class that contains the entry point to the program when your project includes more than one type with a `Main` method. When there's exactly one acceptable `Main` method in the project, this option is ignored.
 
 ```xml
 <StartupObject>MyNamespace.Program</StartupObject>
 ```
 
-Where `Program` is the type that contains the `Main` method. The class name can be either fully qualified (including the namespace) or just the class name if it's unambiguous. For example, when the `Main` method is located inside the `Program` class in the `MyApplication.Core` namespace, you can specify either `-main:MyApplication.Core.Program` or just `-main:Program` if there's no ambiguity. If your compilation includes more than one type with a [`Main`](../../fundamentals/program-structure/main-command-line.md) method, you can specify which type contains the `Main` method.
+Where `Program` is the type that contains the `Main` method. When this option is required (multiple `Main` methods exist), the provided class name must be fully qualified; it must include the full namespace containing the class, followed by the class name. For example, when the `Main` method is located inside the `Program` class in the `MyApplication.Core` namespace, the compiler option has to be `-main:MyApplication.Core.Program`. If the class is in the global namespace, the fully qualified name is the same as the simple class name (for example, `Program` instead of `MyNamespace.Program`).
 
 > [!NOTE]
 > This option can't be used for a project that includes [top-level statements](../../fundamentals/program-structure/top-level-statements.md), even if that project contains one or more `Main` methods.