Help: De-duplicate VS instance selection documentation

Add a section to `CMAKE_GENERATOR_INSTANCE` for VS instance selection,
and reference it from the corresponding sections of each VS generator.

3 files changed, 6 insertions, 36 deletions

diff --git a/Help/generator/Visual Studio 15 2017.rst b/Help/generator/Visual Studio 15 2017.rst
index b4d6f6d..912afad 100644
--- a/Help/generator/Visual Studio 15 2017.rst
+++ b/Help/generator/Visual Studio 15 2017.rst
@@ -17,18 +17,8 @@ Instance Selection
 
 .. versionadded:: 3.11
 
-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.
+VS 2017 supports multiple installations on the same machine.  The
+:variable:`CMAKE_GENERATOR_INSTANCE` variable may be used to select one.
 
 Platform Selection
 ^^^^^^^^^^^^^^^^^^
diff --git a/Help/generator/Visual Studio 16 2019.rst b/Help/generator/Visual Studio 16 2019.rst
index 72399e0..6cefe6d 100644
--- a/Help/generator/Visual Studio 16 2019.rst
+++ b/Help/generator/Visual Studio 16 2019.rst
@@ -15,18 +15,8 @@ Powershell, Python, etc.) are not supported.
 Instance Selection
 ^^^^^^^^^^^^^^^^^^
 
-VS 2019 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 ``VS160COMNTOOLS`` 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 2019 supports multiple installations on the same machine.  The
+:variable:`CMAKE_GENERATOR_INSTANCE` variable may be used to select one.
 
 Platform Selection
 ^^^^^^^^^^^^^^^^^^
diff --git a/Help/generator/Visual Studio 17 2022.rst b/Help/generator/Visual Studio 17 2022.rst
index b3f49f3..edf9d60 100644
--- a/Help/generator/Visual Studio 17 2022.rst
+++ b/Help/generator/Visual Studio 17 2022.rst
@@ -15,18 +15,8 @@ Powershell, Python, etc.) are not supported.
 Instance Selection
 ^^^^^^^^^^^^^^^^^^
 
-VS 2022 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 ``VS170COMNTOOLS`` 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 2022 supports multiple installations on the same machine.  The
+:variable:`CMAKE_GENERATOR_INSTANCE` variable may be used to select one.
 
 Platform Selection
 ^^^^^^^^^^^^^^^^^^