BUG: Document internal cache property MODIFIED

All cmake-defined properties should be documented, even if they are
internal.  This fixes the DocTest when CMAKE_STRICT is enabled.

Source/cmCacheManager.cxx

@@ -992,6 +992,13 @@ void cmCacheManager::DefineProperties(cmake *cm)
      "which creates it (set, option, find_library, etc.)."
       );
 
+  cm->DefineProperty
+    ("MODIFIED", cmProperty::CACHE,
+     "Internal management property.  Do not set or get.",
+     "This is an internal cache entry property managed by CMake to "
+     "track interactive user modification of entries.  Ignore it."
+      );
+
   cm->DefineProperty
     ("STRINGS", cmProperty::CACHE,
      "Enumerate possible STRING entry values for GUI selection.",