Help: Document existence of cmake_install.cmake

This file, which is currently undocumented, is useful for external
packaging programs that wish to install only a single component at a
time. This change adds documentation for the file.

Help/command/install.rst

@@ -400,3 +400,26 @@ executable from the installation tree using the imported target name
   those generated by :command:`install_targets`,
   :command:`install_files`, and :command:`install_programs` commands
   is not defined.
+
+Generated Installation Script
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``install()`` command generates a file, ``cmake_install.cmake``, inside
+the build directory, which is used internally by the generated install target
+and by CPack. You can also invoke this script manually with ``cmake -P``. This
+script accepts several variables:
+
+``COMPONENT``
+  Set this variable to install only a single CPack component as opposed to all
+  of them. For example, if you only want to install the ``Development``
+  component, run ``cmake -DCOMPONENT=Development -P cmake_install.cmake``.
+
+``BUILD_TYPE``
+  Set this variable to change the build type if you are using a multi-config
+  generator. For example, to install with the ``Debug`` configuration, run
+  ``cmake -DBUILD_TYPE=Debug -P cmake_install.cmake``.
+
+``DESTDIR``
+  This is an environment variable rather than a CMake variable. It allows you
+  to change the installation prefix on UNIX systems. See :envvar:`DESTDIR` for
+  details.

Help/release/dev/cmake-install-doc.rst

@@ -0,0 +1,7 @@
+cmake-install-doc
+-----------------
+
+* The existence and functionality of the file
+  ``${CMAKE_BINARY_DIR}/cmake_install.cmake`` has now been documented in the
+  :command:`install` documentation so that external packaging software can take
+  advantage of CPack-style component installs.