runtime(doc): clarify the effect of exclusive single char selections

closes: #17410

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 8cce98a..3105a3d 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jun 01
+*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jun 02
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -5042,6 +5042,10 @@
 		- It is evaluated in current window context, which makes a
 		  difference if the buffer is displayed in a window with
 		  different 'virtualedit' or 'list' values.
+		- When specifying an exclusive selection and {pos1} and {pos2}
+		  are equal, the returned list contains a single character as
+		  if selection is inclusive, to match the behavior of an empty
+		  exclusive selection in Visual mode.
 
 		Examples: >
 			:xnoremap <CR>