patch 8.1.0614: placing signs can be complicated

Problem:    Placing signs can be complicated.
Solution:   Add functions for defining and placing signs.  Introduce a group
            name to avoid different plugins using the same signs. (Yegappan
            Lakshmanan, closes #3652)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 1ad044b..343d476 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -983,6 +983,14 @@
 	job_info()		get information about a job
 	job_setoptions()	set options for a job
 
+Signs:						*sign-functions*
+	sign_define()		define or update a sign
+	sign_getdefined()	get a list of defined signs
+	sign_getplaced()	get a list of placed signs
+	sign_place()		place a sign
+	sign_undefine()		undefine a sign
+	sign_unplace()		unplace a sign
+
 Terminal window:				*terminal-functions*
 	term_start()		open a terminal window and run a job
 	term_list()		get the list of terminal buffers