Help: Factor out VS Build Number components document fragment

Help/variable/CMAKE_GENERATOR_INSTANCE.rst

@@ -43,24 +43,8 @@ Supported pairs are:
   .. versionadded:: 3.23
 
   Specify the 4-component VS Build Version, a.k.a. Build Number.
-  The components are:
 
-  ``<major>.<minor>``
+  .. include:: CMAKE_VS_VERSION_BUILD_NUMBER_COMPONENTS.txt
-
-    The VS major and minor version numbers.
-    These are the same as the release version numbers.
-
-  ``<date>``
-
-    A build date in the format ``MMMDD``, where ``MMM`` is a month index
-    since an epoch used by Microsoft, and ``DD`` is a day in that month.
-
-  ``<build>``
-
-    A build index on the day represented by ``<date>``.
-
-  The build number is reported by ``vswhere`` as ``installationVersion``.
-  For example, VS 16.11.10 has build number ``16.11.32126.315``.
 
 .. versionadded:: 3.23
 

Help/variable/CMAKE_VS_VERSION_BUILD_NUMBER_COMPONENTS.txt

@@ -0,0 +1,18 @@
+The components are:
+
+``<major>.<minor>``
+
+  The VS major and minor version numbers.
+  These are the same as the release version numbers.
+
+``<date>``
+
+  A build date in the format ``MMMDD``, where ``MMM`` is a month index
+  since an epoch used by Microsoft, and ``DD`` is a day in that month.
+
+``<build>``
+
+  A build index on the day represented by ``<date>``.
+
+The build number is reported by ``vswhere`` as ``installationVersion``.
+For example, VS 16.11.10 has build number ``16.11.32126.315``.