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>

Author: chrisbra

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 @@ getregion({pos1}, {pos2} [, {opts}])			*getregion()*
 		- 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>

runtime/doc/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Jun 01
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jun 02
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -7199,9 +7199,14 @@ A jump table for the options with a short description can be found at |Q_op|.
 	the end of line the line break still isn't included.
 	When "exclusive" is used, cursor position in visual mode will be
 	adjusted for inclusive motions |inclusive-motion-selection-exclusive|.
-	Note that when "exclusive" is used and selecting from the end
-	backwards, you cannot include the last character of a line, when
-	starting in Normal mode and 'virtualedit' empty.
+
+	Note:
+	- When "exclusive" is used and selecting from the end backwards, you
+	  cannot include the last character of a line, when starting in Normal
+	  mode and 'virtualedit' empty.
+	- when "exclusive" is used with a single character visual selection,
+	  Vim will behave as if the 'selection' is inclusive (in other words,
+	  you cannot visually select an empty region).
 
 	The 'selection' option is set by the |:behave| command.