patch 9.1.0540: Unused assignment in sign_define_cmd()

Problem:  Unused assignment in sign_define_cmd()
Solution: Remove the assignment.  Also document the "priority" flag of
          sign_define(). (zeertzjq)

closes: #15169

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/sign.txt b/runtime/doc/sign.txt
index 84b2977..679099c 100644
--- a/runtime/doc/sign.txt
+++ b/runtime/doc/sign.txt
@@ -1,4 +1,4 @@
-*sign.txt*      For Vim version 9.1.  Last change: 2024 Jul 06
+*sign.txt*      For Vim version 9.1.  Last change: 2024 Jul 07
 
 
 		  VIM REFERENCE MANUAL    by Gordon Prieur
@@ -404,6 +404,7 @@
 		   icon		full path to the bitmap file for the sign.
 		   linehl	highlight group used for the whole line the
 				sign is placed in.
+		   priority	default priority value of the sign
 		   numhl	highlight group used for the line number where
 				the sign is placed.
 		   text		text that is displayed when there is no icon