Help: Document existence of CMAKE_CPACK_COMMAND

This useful variable was previously undocumented. This commit adds
brief documentation for it.

Help/manual/cmake-variables.7.rst

@@ -24,6 +24,7 @@ Variables that Provide Information
    /variable/CMAKE_CACHE_PATCH_VERSION
    /variable/CMAKE_CFG_INTDIR
    /variable/CMAKE_COMMAND
+   /variable/CMAKE_CPACK_COMMAND
    /variable/CMAKE_CROSSCOMPILING
    /variable/CMAKE_CROSSCOMPILING_EMULATOR
    /variable/CMAKE_CTEST_COMMAND

Help/variable/CMAKE_CPACK_COMMAND.rst

@@ -0,0 +1,8 @@
+CMAKE_CPACK_COMMAND
+-------------------
+
+Full path to :manual:`cpack(1)` command installed with CMake.
+
+This is the full path to the CPack executable :manual:`cpack(1)` which is
+useful from custom commands that want to use the :manual:`cmake(1)` ``-E``
+option for portable system commands.