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.
diff --git a/runtime/doc/sign.txt b/runtime/doc/sign.txt
index 4f59c8f..0903976 100644
--- a/runtime/doc/sign.txt
+++ b/runtime/doc/sign.txt
@@ -182,9 +182,9 @@
 
 		By default, the sign is assigned a default priority of 10. To
 		assign a different priority value, use "priority={prio}" to
-		specify a value. The priority is used to determine the
-		highlight group used when multiple signs are placed on the
-		same line.
+		specify a value.  The priority is used to determine the sign
+		that is displayed when multiple signs are placed on the same
+		line.
 
 		Examples: >
 			:sign place 5 line=3 name=sign1 file=a.py
@@ -204,7 +204,9 @@
 		it (e.g., when the debugger has stopped at a breakpoint).
 
 		The optional "group={group}" attribute can be used before
-		"file=" to select a sign in a particular group.
+		"file=" to select a sign in a particular group.  The optional
+		"priority={prio}" attribute can be used to change the priority
+		of an existing sign.
 
 :sign place {id} name={name} [buffer={nr}]
 		Same, but use buffer {nr}.  If the buffer argument is not