Help: Document the CMAKE_MAKE_PROGRAM variable in more detail

Explain how it is set for each group of generators.  Also explain
the build-time selection behavior used by Visual Studio generators.

Help/variable/CMAKE_MAKE_PROGRAM.rst

@@ -1,10 +1,51 @@
 CMAKE_MAKE_PROGRAM
 ------------------
 
-Tool used for the actual build process.
+Tool that can launch the native build system.
+The value may be the full path to an executable or just the tool
+name if it is expected to be in the ``PATH``.
 
-This variable is set to the program that will be needed to build the
-output of CMake.  If the generator selected was Visual Studio 6, the
-CMAKE_MAKE_PROGRAM will be set to msdev, for Unix Makefiles it will be
-set to make or gmake, and for Visual Studio 7 it set to devenv.  For
-NMake Makefiles the value is nmake.
+The tool selected depends on the :variable:`CMAKE_GENERATOR` used
+to configure the project:
+
+* The Makefile generators set this to ``make``, ``gmake``, or
+  a generator-specific tool (e.g. ``nmake`` for "NMake Makefiles").
+
+  These generators store ``CMAKE_MAKE_PROGRAM`` in the CMake cache
+  so that it may be edited by the user.
+
+* The Ninja generator sets this to ``ninja``.
+
+  This generator stores ``CMAKE_MAKE_PROGRAM`` in the CMake cache
+  so that it may be edited by the user.
+
+* The Xcode generator sets this to ``xcodebuild`` (or possibly an
+  otherwise undocumented ``cmakexbuild`` wrapper implementing some
+  workarounds).
+
+  This generator stores ``CMAKE_MAKE_PROGRAM`` in the CMake cache
+  so that it may be edited by the user.
+
+* The Visual Studio generators set this to the full path to
+  ``MSBuild.exe`` (VS >= 10), ``devenv.com`` (VS 7,8,9),
+  ``VCExpress.exe`` (VS Express 8,9), or ``msdev.exe`` (VS 6).
+
+  These generators prefer to lookup the build tool at build time
+  rather than to store ``CMAKE_MAKE_PROGRAM`` in the CMake cache
+  ahead of time.  This is because the tools are version-specific
+  and can be located using the Windows Registry.  It is also
+  necessary because the proper build tool may depend on the
+  project content (e.g. the Intel Fortran plugin to VS 10 and 11
+  requires ``devenv.com`` to build its ``.vfproj`` project files
+  even though ``MSBuild.exe`` is normally preferred to support
+  the :variable:`CMAKE_GENERATOR_TOOLSET`).
+
+  For compatibility with versions of CMake prior to 3.0, if
+  a user or project explicitly adds ``CMAKE_MAKE_PROGRAM`` to
+  the CMake cache then CMake will use the specified value if
+  possible.
+
+The ``CMAKE_MAKE_PROGRAM`` variable is set for use by project code.
+The value is also used by the :manual:`cmake(1)` ``--build`` and
+:manual:`ctest(1)` ``--build-and-test`` tools to launch the native
+build process.