patch 9.0.1212: cannot read back what setcellwidths() has done

Problem:    Cannot read back what setcellwidths() has done.
Solution:   Add getcellwidths(). (Kota Kato, closes #11837)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 5597077..7a706af 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -757,6 +757,7 @@
 	strwidth()		size of string when displayed
 	strdisplaywidth()	size of string when displayed, deals with tabs
 	setcellwidths()		set character cell width overrides
+	getcellwidths()		get 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