patch 8.1.1682: placing a larger number of signs is slow

Problem:    Placing a larger number of signs is slow.
Solution:   Add functions for dealing with a list of signs. (Yegappan
            Lakshmanan, closes #4636)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 34d60f1..0f677e4 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1015,8 +1015,10 @@
 	sign_getplaced()	get a list of placed signs
 	sign_jump()		jump to a sign
 	sign_place()		place a sign
+	sign_placelist()	place a list of signs
 	sign_undefine()		undefine a sign
 	sign_unplace()		unplace a sign
+	sign_unplacelist()	unplace a list of signs
 
 Terminal window:				*terminal-functions*
 	term_start()		open a terminal window and run a job