patch 9.1.0430: getregionpos() doesn't handle one char selection

Problem:  getregionpos() doesn't handle one char selection.
Solution: Handle startspaces differently when is_oneChar is set.
          Also add a test for an exclusive charwise selection with
          multibyte chars (zeertzjq)

closes: #14825

Signed-off-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 b9dd4d2..4177a01 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 20
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 May 22
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -4345,8 +4345,8 @@
 		the offset in screen columns from the start of the character.
 		E.g., a position within a <Tab> or after the last character.
 		If the "off" number of an ending position is non-zero, it is
-		the character's number of cells included in the selection,
-		otherwise the whole character is included.
+		the offset of the character's first cell not included in the
+		selection, otherwise all its cells are included.
 
 		Can also be used as a |method|: >
 			getpos('.')->getregionpos(getpos("'a"))