patch 8.0.0580: cannot set the valid flag with setqflist()

Problem:    Cannot set the valid flag with setqflist().
Solution:   Add the "valid" argument. (Yegappan Lakshmanan, closes #1642)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 83bc8f3..20297f5 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -6945,6 +6945,7 @@
 		    nr		error number
 		    text	description of the error
 		    type	single-character error type, 'E', 'W', etc.
+		    valid	recognized error message
 
 		The "col", "vcol", "nr", "type" and "text" entries are
 		optional.  Either "lnum" or "pattern" entry can be used to
@@ -6954,6 +6955,8 @@
 		item will not be handled as an error line.
 		If both "pattern" and "lnum" are present then "pattern" will
 		be used.
+		If the "valid" entry is not supplied, then the valid flag is
+		set when "bufnr" is a valid buffer or "filename" exists.
 		If you supply an empty {list}, the quickfix list will be
 		cleared.
 		Note that the list is not exactly the same as what