patch 9.1.1507: symlinks are resolved on :cd commands

Problem:  File paths change from symlink to target path after :cd command
          when editing files through symbolic links
Solution: Add "~" flag to 'cpoptions' to control symlink resolution.
          When not included (default), symlinks are resolved maintaining
          backward compatibility. When included, symlinks are preserved
          providing the improved behavior. (glepnir)

related: neovim/neovim#15695
closes: #17628

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index b3c5697..cf064e1 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Jul 01
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jul 03
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2390,7 +2390,7 @@
 
 						*'cpoptions'* *'cpo'* *cpo*
 'cpoptions' 'cpo'	string	(Vim default: "aABceFsz",
-				 Vi default:  all flags, except "#{|&/\."
+				 Vi default:  all flags, except "#{|&/\.~"
 				 |$VIM_POSIX|:  all flags)
 			global
 	A sequence of single character flags.  When a character is present
@@ -2680,6 +2680,13 @@
 			character, the cursor won't move. When not included,
 			the cursor would skip over it and jump to the
 			following occurrence.
+								*cpo-~*
+		~	When included, don't resolve symbolic links when
+			changing directory with |:cd|, |:lcd|, or |:tcd|.
+			This preserves the symbolic link path in buffer names
+			and when displaying the current directory.  When
+			excluded (default), symbolic links are resolved to
+			their target paths.
 
 	POSIX flags.  These are not included in the Vi default value, except
 	when $VIM_POSIX was set on startup. |posix|