improve doc about catkin_package(CFG_EXTRAS) (#805)

cmake/catkin_package.cmake

@@ -36,8 +36,10 @@
 # :param CFG_EXTRAS: a CMake file containing extra stuff that should
 #   be accessible to users of this package after
 #   ``find_package``\ -ing it.  This file must live in the
-#   subdirectory ``cmake`` or be an absolute path.  Various additional
-#   file extension are possible:
+#   subdirectory ``cmake`` or be an absolute path.
+#   All passed extra files must have unique basenames since they are
+#   being installed into a single folder.
+#   Various additional file extension are possible:
 #   for a plain cmake file just ``.cmake``, for files expanded using
 #   CMake's ``configure_file()`` use ``.cmake.in`` or for files expanded
 #   by empy use ``.cmake.em``.  The templates can distinguish between