Help: Clarify SYSTEM property default for imported targets

The `SYSTEM` directory property does not affect imported targets.

Fixes: #24219

Help/command/add_subdirectory.rst

@@ -36,7 +36,7 @@ the dependency.
 
 If the ``SYSTEM`` argument is provided, the :prop_dir:`SYSTEM` directory
 property of the subdirectory will be set to true.  This property is
-used to initialize the :prop_tgt:`SYSTEM` property of each target
-created in that subdirectory. The include directories of targets with
-:prop_tgt:`SYSTEM` set to true will be treated as ``SYSTEM`` when
+used to initialize the :prop_tgt:`SYSTEM` property of each non-imported
+target created in that subdirectory.  The include directories of targets
+with :prop_tgt:`SYSTEM` set to true will be treated as ``SYSTEM`` when
 compiling consumers.

Help/prop_dir/SYSTEM.rst

@@ -4,7 +4,7 @@ SYSTEM
 .. versionadded:: 3.25
 
 This directory property is used to initialize the :prop_tgt:`SYSTEM`
-target property for targets created in that directory. It is set to
-true by :command:`add_subdirectory` and
+target property for non-imported targets created in that directory.
+It is set to true by :command:`add_subdirectory` and
 :command:`FetchContent_Declare` when the ``SYSTEM`` option is given
 as an argument to those commands.

Help/prop_tgt/SYSTEM.rst

@@ -18,5 +18,5 @@ then their :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` will not be
 treated as ``SYSTEM``, regardless of the value of the
 :prop_tgt:`IMPORTED_NO_SYSTEM` property.
 
-This target property is initialized from the :prop_dir:`SYSTEM`
-directory property when the target is created.
+For non-imported targets, this target property is initialized from
+the :prop_dir:`SYSTEM` directory property when the target is created.