patch 8.2.1535: it is not possible to specify cell widths of characters

Problem:    It is not possible to specify cell widths of characters.
Solution:   Add setcellwidths().
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 459f6b0..bcd58c9 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -611,6 +611,7 @@
 	strchars()		length of a string in characters
 	strwidth()		size of string when displayed
 	strdisplaywidth()	size of string when displayed, deals with tabs
+	setcellwidths()		set character cell width overrides
 	substitute()		substitute a pattern match with a string
 	submatch()		get a specific match in ":s" and substitute()
 	strpart()		get part of a string using byte index