patch 8.1.0644: finding next sign ID is inefficient

Problem:    Finding next sign ID is inefficient.
Solution:   Add next_sign_id. (Yegappan Lakshmanan, closes #3717)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index cb1c61b..85b9e44 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -7936,10 +7936,10 @@
 			lnum	select signs placed in this line. For the use
 				of {lnum}, see |line()|.
 		If {group} is '*', then signs in all the groups including the
-		global group are returned. If {group} is not supplied, then
-		only signs in the global group are returned.  If no arguments
-		are supplied, then signs in the global group placed in all the
-		buffers are returned.
+		global group are returned. If {group} is not supplied or is an
+		empty string, then only signs in the global group are
+		returned.  If no arguments are supplied, then signs in the
+		global group placed in all the buffers are returned.
 
 		Each list item in the returned value is a dictionary with the
 		following entries: