runtime(doc): clarify temporary file clean up

related: #14770

Co-authored-by: Enno <Konfekt@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 1d06e75..46819d6 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 May 15
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 May 18
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -10243,7 +10243,9 @@
 		is different for at least 26 consecutive calls.  Example: >
 			:let tmpfile = tempname()
 			:exe "redir > " .. tmpfile
-<		For Unix, the file will be in a private directory |tempfile|.
+<		For Unix, the file will be in a private directory |tempfile|
+		that is recursively deleted when Vim exits, on other systems
+		temporary files are not cleaned up automatically on exit.
 		For MS-Windows forward slashes are used when the 'shellslash'
 		option is set, or when 'shellcmdflag' starts with '-' and
 		'shell' does not contain powershell or pwsh.