patch 8.1.0717: there is no function for the ":sign jump" command

Problem:    There is no function for the ":sign jump" command.
Solution:   Add the sign_jump() function. (Yegappan Lakshmanan, closes #3780)
diff --git a/runtime/doc/sign.txt b/runtime/doc/sign.txt
index 6eb11a0..7b33a2d 100644
--- a/runtime/doc/sign.txt
+++ b/runtime/doc/sign.txt
@@ -263,13 +263,13 @@
 		all the files it appears in.
 
 :sign unplace *
-		Remove placed signs in the global group from all the files.
+		Remove all placed signs in the global group from all the files.
 
 :sign unplace * group={group}
-		Remove placed signs in group {group} from all the files.
+		Remove all placed signs in group {group} from all the files.
 
 :sign unplace * group=*
-		Remove placed signs in all the groups from all the files.
+		Remove all placed signs in all the groups from all the files.
 
 :sign unplace
 		Remove a placed sign at the cursor position. If multiple signs
@@ -317,6 +317,8 @@
 
 JUMPING TO A SIGN					*:sign-jump* *E157*
 
+See |sign_jump()| for the equivalent Vim script function.
+
 :sign jump {id} file={fname}
 		Open the file {fname} or jump to the window that contains
 		{fname} and position the cursor at sign {id}.