patch 8.2.2324: not easy to get mark en cursor posotion by character count

Problem:    Not easy to get mark en cursor posotion by character count.
Solution:   Add functions that use character index. (Yegappan Lakshmanan,
            closes #7648)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index a035038..da095e1 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -753,6 +753,11 @@
 	screenchar()		get character code at a screen line/row
 	screenchars()		get character codes at a screen line/row
 	screenstring()		get string of characters at a screen line/row
+	charcol()		character number of the cursor or a mark
+	getcharpos()		get character position of cursor, mark, etc.
+	setcharpos()		set character position of cursor, mark, etc.
+	getcursorcharpos()	get character position of the cursor
+	setcursorcharpos()	set character position of the cursor
 
 Working with text in the current buffer:		*text-functions*
 	getline()		get a line or list of lines from the buffer