CheckTypeSize: Add code examples

This adds some basic examples of the `<variable>_CODE` and
`<variable>_KEYS`.

Modules/CheckTypeSize.cmake

@@ -83,6 +83,62 @@ the way the check is run:
 
 ``CMAKE_EXTRA_INCLUDE_FILES``
   list of extra headers to include.
+
+Examples
+^^^^^^^^
+
+Consider the code:
+
+.. code-block:: cmake
+
+  include(CheckTypeSize)
+
+  # Check for size of long.
+  check_type_size(long SIZEOF_LONG)
+  message("HAVE_SIZEOF_LONG: ${HAVE_SIZEOF_LONG}")
+  message("SIZEOF_LONG: ${SIZEOF_LONG}")
+  message("SIZEOF_LONG_CODE: ${SIZEOF_LONG_CODE}")
+
+On a 64-bit architecture, the output may look something like this::
+
+  HAVE_SIZEOF_LONG: TRUE
+  SIZEOF_LONG: 8
+  SIZEOF_LONG_CODE: #define SIZEOF_LONG 8
+
+On Apple platforms, when :variable:`CMAKE_OSX_ARCHITECTURES` has multiple
+architectures, types may have architecture-dependent sizes.
+For example, with the code
+
+.. code-block:: cmake
+
+  include(CheckTypeSize)
+
+  check_type_size(long SIZEOF_LONG)
+  message("HAVE_SIZEOF_LONG: ${HAVE_SIZEOF_LONG}")
+  message("SIZEOF_LONG: ${SIZEOF_LONG}")
+  foreach(key IN LISTS SIZE_OF_LONG_KEYS)
+    message("key: ${key}")
+    message("value: ${SIZE_OF_LONG-${key}}")
+  endforeach()
+  message("SIZEOF_LONG_CODE:
+  ${SIZEOF_LONG_CODE}")
+
+the result may be::
+
+  HAVE_SIZEOF_LONG: TRUE
+  SIZEOF_LONG: 0
+  key: __i386
+  value: 4
+  key: __x86_64
+  value: 8
+  SIZEOF_LONG_CODE:
+  #if defined(__i386)
+  # define SIZE_OF_LONG 4
+  #elif defined(__x86_64)
+  # define SIZE_OF_LONG 8
+  #else
+  # error SIZE_OF_LONG unknown
+  #endif
 #]=======================================================================]
 
 include(CheckIncludeFile)