patch 8.1.0218: cannot add matches to another window

Problem:    Cannot add matches to another window. (Qiming Zhao)
Solution:   Add the "window" argument to matchadd() and matchaddpos().
            (closes #3260)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index e665c35..fb9093e 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -6016,7 +6016,7 @@
 		the pattern.  'smartcase' is NOT used.  The matching is always
 		done like 'magic' is set and 'cpoptions' is empty.
 
-					*matchadd()* *E798* *E799* *E801*
+				*matchadd()* *E798* *E799* *E801* *E957*
 matchadd({group}, {pattern} [, {priority} [, {id} [, {dict}]]])
 		Defines a pattern to be highlighted in the current window (a
 		"match").  It will be highlighted with {group}.  Returns an
@@ -6055,6 +6055,8 @@
 			conceal	    Special character to show instead of the
 				    match (only for |hl-Conceal| highlighted
 				    matches, see |:syn-cchar|)
+			window	    Instead of the current window use the
+				    window with this number or window ID.
 
 		The number of matches is not limited, as it is the case with
 		the |:match| commands.