patch 8.1.2229: cannot color number column above/below cursor differently

Problem:    Cannot color number column above/below cursor differently.
Solution:   Add LineNrAbove and LineNrBelow. (Shaun Brady, closes #624)
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index e45ff81..1c4988a 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -5071,6 +5071,12 @@
 							*hl-LineNr*
 LineNr		Line number for ":number" and ":#" commands, and when 'number'
 		or 'relativenumber' option is set.
+							*hl-LineNrAbove*
+LineNrAbove	Line number for when the 'relativenumber'
+		option is set, above the cursor line.
+							*hl-LineNrBelow*
+LineNrBelow	Line number for when the 'relativenumber'
+		option is set, below the cursor line.
 							*hl-CursorLineNr*
 CursorLineNr	Like LineNr when 'cursorline' is set and 'cursorlineopt' is
 		set to "number" or "both", or 'relativenumber' is set, for