patch 8.1.1466: not updating priority on existing sign

Problem:    Not updating priority on existing sign.
Solution:   Set the sign priority.  Add a test. (Yegappan Lakshmanan)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index c095d58..21beaff 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -8597,7 +8597,7 @@
 			priority	sign priority
 
 		The returned signs in a buffer are ordered by their line
-		number.
+		number and priority.
 
 		Returns an empty list on failure or if there are no placed
 		signs.