patch 8.2.2233: cannot convert a byte index into a character index

Problem:    Cannot convert a byte index into a character index.
Solution:   Add charidx(). (Yegappan Lakshmanan, closes #7561)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index a19d005..a035038 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -625,6 +625,7 @@
 	iconv()			convert text from one encoding to another
 	byteidx()		byte index of a character in a string
 	byteidxcomp()		like byteidx() but count composing characters
+	charidx()		character index of a byte in a string
 	repeat()		repeat a string multiple times
 	eval()			evaluate a string expression
 	execute()		execute an Ex command and get the output