runtime(doc): 'cpoptions': Include "z" in the documented default

A fixup for commit 22105fd1fe0dcfe993b5c04 (tag: v9.1.0589)

While at it, document the correct Vi defaults and $VIM_POSIX (even more
compatible) flags.

closes: #15274

Signed-off-by: Illia Bobyr <illia.bobyr@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index b211dcb..3021401 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2024 Jul 15
+*options.txt*	For Vim version 9.1.  Last change: 2024 Jul 16
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2257,8 +2257,9 @@
 	Also see 'preserveindent'.
 
 						*'cpoptions'* *'cpo'* *cpo*
-'cpoptions' 'cpo'	string	(Vim default: "aABceFs",
-				 Vi default:  all flags)
+'cpoptions' 'cpo'	string	(Vim default: "aABceFsz",
+				 Vi default:  all flags, except "#{|&/\."
+                                 |$VIM_POSIX|:  all flags)
 			global
 	A sequence of single character flags.  When a character is present
 	this indicates Vi-compatible behavior.  This is used for things where