Update windows_subsystem to use the attribute template

Author: ehuss

src/runtime.md

@@ -13,18 +13,27 @@ r[runtime.windows_subsystem]
 ## The `windows_subsystem` attribute
 
 r[runtime.windows_subsystem.intro]
-The *`windows_subsystem` attribute* may be applied at the crate level to set the [subsystem] when linking on a Windows target.
+The *`windows_subsystem` [attribute][attributes]* sets the [subsystem] when linking on a Windows target.
 
 > [!EXAMPLE]
 > ```rust
 > #![windows_subsystem = "windows"]
 > ```
 
 r[runtime.windows_subsystem.syntax]
-It uses the [MetaNameValueStr] syntax to specify the subsystem with a value of either `console` or `windows`.
+The `windows_subsystem` attribute uses the [MetaNameValueStr] syntax to specify the subsystem with a value of either `console` or `windows`.
+
+r[runtime.windows_subsystem.allowed-positions]
+The `windows_subsystem` attribute may only be applied to the crate root.
+
+r[runtime.windows_subsystem.duplicates]
+Only the first instance of `windows_subsystem` on an item is honored. Subsequent `example` attributes are ignored.
+
+> [!NOTE]
+> `rustc` currently warns on subsequent duplicate `example` attributes. This may become an error in the future.
 
 r[runtime.windows_subsystem.ignored]
-This attribute is ignored on non-Windows targets, and for non-`bin` [crate types].
+The `windows_subsystem` attribute is ignored on non-Windows targets, and for non-`bin` [crate types].
 
 r[runtime.windows_subsystem.console]
 The "console" subsystem is the default. If a console process is run from an existing console then it will be attached to that console, otherwise a new console window will be created.