Help: Fix versionadded for VS 15 2017 instance selection

Support for `CMAKE_GENERATOR_INSTANCE` was added in CMake 3.11, but the
possibility was mentioned in a comment in older versions, so the wrong
versionadded value was used in commit c43e845d09 (Help: Add `..
versionadded` directives to generator docs, 2020-11-11,
v3.20.0-rc1~476^2).

Help/generator/Visual Studio 15 2017.rst

@@ -15,20 +15,20 @@ Powershell, Python, etc.) are not supported.
 Instance Selection
 ^^^^^^^^^^^^^^^^^^
 
-.. versionadded:: 3.9
-  VS 2017 supports multiple installations on the same machine.
-  The :variable:`CMAKE_GENERATOR_INSTANCE` variable may be set as a
-  cache entry containing the absolute path to a Visual Studio instance.
-  If the value is not specified explicitly by the user or a toolchain file,
-  CMake queries the Visual Studio Installer to locate VS instances, chooses
-  one, and sets the variable as a cache entry to hold the value persistently.
-
 .. versionadded:: 3.11
-  When CMake first chooses an instance, if the ``VS150COMNTOOLS`` environment
-  variable is set and points to the ``Common7/Tools`` directory within
-  one of the instances, that instance will be used.  Otherwise, if more
-  than one instance is installed we do not define which one is chosen
-  by default.
+
+VS 2017 supports multiple installations on the same machine.
+The :variable:`CMAKE_GENERATOR_INSTANCE` variable may be set as a
+cache entry containing the absolute path to a Visual Studio instance.
+If the value is not specified explicitly by the user or a toolchain file,
+CMake queries the Visual Studio Installer to locate VS instances, chooses
+one, and sets the variable as a cache entry to hold the value persistently.
+
+When CMake first chooses an instance, if the ``VS150COMNTOOLS`` environment
+variable is set and points to the ``Common7/Tools`` directory within
+one of the instances, that instance will be used.  Otherwise, if more
+than one instance is installed we do not define which one is chosen
+by default.
 
 Platform Selection
 ^^^^^^^^^^^^^^^^^^