Help: Clarify which items are retained by $<REMOVE_DUPLICATES:...>

Help/manual/cmake-generator-expressions.7.rst

diff --git a/Help/manual/cmake-generator-expressions.7.rst b/Help/manual/cmake-generator-expressions.7.rst
index 1ef1ec8..22c14df 100644
--- a/Help/manual/cmake-generator-expressions.7.rst
+++ b/Help/manual/cmake-generator-expressions.7.rst
@@ -599,7 +599,9 @@
 
   .. versionadded:: 3.15
 
-  Removes duplicated items in the given ``list``.
+  Removes duplicated items in the given ``list``. The relative order of items
+  is preserved, but if duplicates are encountered, only the first instance is
+  preserved.
 
 .. genex:: $<FILTER:list,INCLUDE|EXCLUDE,regex>