runtime(doc): clarify documentation for "v" position at line()

Problem: the previous documentation falsely states that "v" always
refers to the start of a visual area.  In fact, the reference of "v" and
"." complement each other.  If the cursor is at the start of
a (characterwise) visual area, then "v" refers to the end of the area.

Solution: be more verbose and explicit about the connection between "."
and "v" and also refer to |v_o| which many vim users will be familiar
with for visual areas.

Signed-off-by: Peter Aronoff <peter@aronoff.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 2ed94ea..66f1bae 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 Jun 06
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 Jun 11
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -6191,10 +6191,21 @@
 			    display isn't updated, e.g. in silent Ex mode)
 		    w$	    last line visible in current window (this is one
 			    less than "w0" if no lines are visible)
-		    v	    In Visual mode: the start of the Visual area (the
-			    cursor is the end).  When not in Visual mode
-			    returns the cursor position.  Differs from |'<| in
-			    that it's updated right away.
+		    v	    When not in Visual mode, returns the cursor
+			    position.  In Visual mode, returns the other end
+			    of the Visual area.  A good way to think about
+			    this is that in Visual mode "v" and "." complement
+			    each other.  While "." refers to the cursor
+			    position, "v" refers to where |v_o| would move the
+			    cursor.  As a result, you can use "v" and "."
+			    together to work on all of a selection in
+			    characterwise visual mode.  If the cursor is at
+			    the end of a characterwise visual area, "v" refers
+			    to the start of the same visual area.  And if the
+			    cursor is at the start of a characterwise visual
+			    area, "v" refers to the end of the same visual
+			    area.  "v" differs from |'<| and |'>| in that it's
+			    updated right away.
 		Note that a mark in another file can be used.  The line number
 		then applies to another buffer.
 		To get the column number use |col()|.  To get both use