patch 8.1.1071: cannot get composing characters from the screen

Problem:    Cannot get composing characters from the screen.
Solution:   Add screenchars() and screenstring(). (partly by Ozaki Kiichi,
            closes #4059)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 31beded6..faa8b6b 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -723,6 +723,8 @@
 	diff_filler()		get the number of filler lines above a line
 	screenattr()		get attribute at a screen line/row
 	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
 
 Working with text in the current buffer:		*text-functions*
 	getline()		get a line or list of lines from the buffer