patch 8.2.3874: cannot highlight the number column for a sign

Problem:    Cannot highlight the number column for a sign.
Solution:   Add the "numhl" argument. (James McCoy, closes #9381)
diff --git a/runtime/doc/sign.txt b/runtime/doc/sign.txt
index 7976cec..125e592 100644
--- a/runtime/doc/sign.txt
+++ b/runtime/doc/sign.txt
@@ -140,6 +140,11 @@
 		Highlighting group used for the whole line the sign is placed
 		in.  Most useful is defining a background color.
 
+	numhl={group}
+		Highlighting group used for the line number on the line where
+		the sign is placed.  Overrides |hl-LineNr|, |hl-LineNrAbove|,
+		|hl-LineNrBelow|, and |hl-CursorLineNr|.
+
 	text={text}						*E239*
 		Define the text that is displayed when there is no icon or the
 		GUI is not being used.  Only printable characters are allowed
@@ -396,6 +401,8 @@
 		   icon		full path to the bitmap file for the sign.
 		   linehl	highlight group used for the whole line the
 				sign is placed in.
+		   numhl	highlight group used for the line number where
+				the sign is placed.
 		   text		text that is displayed when there is no icon
 				or the GUI is not being used.
 		   texthl	highlight group used for the text item
@@ -443,6 +450,8 @@
 		   linehl	highlight group used for the whole line the
 				sign is placed in; not present if not set
 		   name		name of the sign
+		   numhl	highlight group used for the line number where
+				the sign is placed; not present if not set
 		   text		text that is displayed when there is no icon
 				or the GUI is not being used.
 		   texthl	highlight group used for the text item; not