Help: Document SYSTEM treatment of IMPORTED target INTERFACE_INCLUDE_DIRS

Document how the behavior can be controlled.

Help/manual/cmake-buildsystem.7.rst

@@ -602,6 +602,14 @@ This is equivalent to appending ``${CMAKE_INSTALL_PREFIX}/include`` to the
 :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` of each of the installed
 :prop_tgt:`IMPORTED` targets when generated by :command:`install(EXPORT)`.
 
+When the :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` of an
+:ref:`imported target <Imported targets>` is consumed, the entries in the
+property are treated as ``SYSTEM`` include directories, as if they were
+listed in the :prop_tgt:`INTERFACE_SYSTEM_INCLUDE_DIRECTORIES` of the
+dependency. This can result in omission of compiler warnings for headers
+found in those directories.  This behavior for :ref:`imported targets` may
+be controlled with the :prop_tgt:`NO_SYSTEM_FROM_IMPORTED` target property.
+
 If a binary target is linked transitively to a Mac OX framework, the
 ``Headers`` directory of the framework is also treated as a usage requirement.
 This has the same effect as passing the framework directory as an include