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.

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
 ^^^^^^^^^^^^^^^^^^

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
 ^^^^^^^^^^^^^^^^^^

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
 ^^^^^^^^^^^^^^^^^^

Help/variable/CMAKE_GENERATOR_INSTANCE.rst

@@ -18,10 +18,21 @@ variable may initialize ``CMAKE_GENERATOR_INSTANCE`` as a cache entry.
 Once a given build tree has been initialized with a particular value
 for this variable, changing the value has undefined behavior.
 
-Instance specification is supported only on specific generators:
+Instance specification is supported only on specific generators.
 
-* For the :generator:`Visual Studio 15 2017` generator (and above)
-  this specifies the absolute path to the VS installation directory
-  of the selected VS instance.
+Visual Studio Instance Selection
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-See native build system documentation for allowed instance values.
+:ref:`Visual Studio Generators` support instance specification for
+Visual Studio 2017 and above.  The ``CMAKE_GENERATOR_INSTANCE`` variable
+may be set as a cache entry selecting an instance of Visual Studio
+via the absolute path to the top-level directory of the VS installation.
+
+If the value of ``CMAKE_GENERATOR_INSTANCE`` 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.  If an environment variable of the form
+``VS##0COMNTOOLS``, where ``##`` the Visual Studio major version number,
+is set and points to the ``Common7/Tools`` directory within one of the
+VS instances, that instance will be used.  Otherwise, if more than one
+VS instance is installed we do not define which one is chosen by default.