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/src/sign.c b/src/sign.c
index 1ae714d..b1f496c 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -1390,7 +1390,6 @@
 	{
 	    arg += 9;
 	    prio = atoi((char *)arg);
-	    arg = skiptowhite(arg);
 	}
 	else
 	{
diff --git a/src/version.c b/src/version.c
index 456f2d6..da584b9 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    540,
+/**/
     539,
 /**/
     538,