ExternalProject: reword `LIST_SEPARATOR` to indicate what it *does*

Also remove the (IMO) confusing suggestion to have ` ` as a separator as
it interferes with things like spaces in argument values (e.g., paths)
or generated arguments such as `-GUnix Makefiles`. The new example is
likely more common usage of the facility.

Modules/ExternalProject.cmake

@@ -906,13 +906,13 @@ Miscellaneous Options
 
 ``LIST_SEPARATOR <sep>``
   For any of the various ``..._COMMAND`` options, and ``CMAKE_ARGS``,
-  replace ``;`` with ``<sep>`` in the specified command lines.
-  This can be useful where list variables may be given in commands where
-  they should end up as space-separated arguments (``<sep>`` would be a
-  single space character string in this case). Note that the separator should
-  be chosen to avoid being confused for non-list-separator usages of the
-  sequence. For example, using ``LIST_SEPARATOR`` allows for passing list
-  values to CMake cache variables on the command line:
+  ``ExternalProject`` will replace ``<sep>`` with ``;`` in the specified
+  command lines. This can be used to ensure a command has a literal ``;`` in it
+  where direct usage would otherwise be interpreted as argument separators to
+  CMake APIs instead. Note that the separator should be chosen to avoid being
+  confused for non-list-separator usages of the sequence. For example, using
+  ``LIST_SEPARATOR`` allows for passing list values to CMake cache variables on
+  the command line:
 
   .. code-block:: cmake