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/options.txt b/runtime/doc/options.txt
index 82cac9d..1ef944d 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3923,6 +3923,7 @@
 				     "8:SpecialKey,~:EndOfBuffer,@:NonText,
 				     d:Directory,e:ErrorMsg,i:IncSearch,
 				     l:Search,m:MoreMsg,M:ModeMsg,n:LineNr,
+				     a:LineNrAbove,b:LineNrBelow,
 				     N:CursorLineNr,r:Question,s:StatusLine,
 				     S:StatusLineNC,c:VertSplit,t:Title,
 				     v:Visual,V:VisualNOS,w:WarningMsg,
@@ -3954,6 +3955,10 @@
 	|hl-ModeMsg|	 M  Mode (e.g., "-- INSERT --")
 	|hl-LineNr|	 n  line number for ":number" and ":#" commands, and
 			    when 'number' or 'relativenumber' option is set.
+	|hl-LineNrAbove|   a  line number above the cursor for when the
+			    'relativenumber' option is set.
+	|hl-LineNrBelow|   b  line number below the cursor for when the
+			    'relativenumber' option is set.
 	|hl-CursorLineNr|  N like n for when 'cursorline' or 'relativenumber' is
 			    set.
 	|hl-Question|	 r  |hit-enter| prompt and yes/no questions
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