Help: Improve CMAKE_SYSTEM_{NAME,VERSION} variable documentation

Specify their meaning for host builds and when cross compiling.
Sepcify their relationship in each case.

1 files changed, 16 insertions, 4 deletions

diff --git a/Help/variable/CMAKE_SYSTEM_NAME.rst b/Help/variable/CMAKE_SYSTEM_NAME.rst
index 0466da4..c3a42e5 100644
--- a/Help/variable/CMAKE_SYSTEM_NAME.rst
+++ b/Help/variable/CMAKE_SYSTEM_NAME.rst
@@ -1,8 +1,20 @@
 CMAKE_SYSTEM_NAME
 -----------------
 
-Name of the operating system CMake is building for.
+The name of the operating system for which CMake is to build.
+See the :variable:`CMAKE_SYSTEM_VERSION` variable for the OS version.
 
-This is the name of the operating system on which CMake is targeting.  This
-variable is the same as :variable:`CMAKE_HOST_SYSTEM_NAME` if you build for the
-host system instead of the target system when cross compiling.
+System Name for Host Builds
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``CMAKE_SYSTEM_NAME`` is by default set to the same value as the
+:variable:`CMAKE_HOST_SYSTEM_NAME` variable so that the build
+targets the host system.
+
+System Name for Cross Compiling
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``CMAKE_SYSTEM_NAME`` may be set explicitly when first configuring a new build
+tree in order to enable :ref:`cross compiling <Cross Compiling Toolchain>`.
+In this case the :variable:`CMAKE_SYSTEM_VERSION` variable must also be
+set explicitly.