patch 9.1.0771: completion attribute hl_group is confusing

Problem:  Currently completion attribute hl_group is combined with
          all items, which is redundant and confusing with kind_hlgroup
Solution: Renamed to abbr_hlgroup and combine it only with the abbr item
          (glepnir).

closes: #15818

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/insert.txt b/runtime/doc/insert.txt
index a954eef..839d17a 100644
--- a/runtime/doc/insert.txt
+++ b/runtime/doc/insert.txt
@@ -1183,12 +1183,12 @@
 	user_data	custom data which is associated with the item and
 			available in |v:completed_item|; it can be any type;
 			defaults to an empty string
-	hl_group	an additional highlight group whose attributes are
+	abbr_hlgroup	an additional highlight group whose attributes are
 			combined with |hl-PmenuSel| and |hl-Pmenu| or
 			|hl-PmenuMatchSel| and |hl-PmenuMatch| highlight
 			attributes in the popup menu to apply cterm and gui
 			properties (with higher priority) like strikethrough
-			to the completion items
+			to the completion items abbreviation
 	kind_hlgroup	an additional highlight group specifically for setting
 			the highlight attributes of the completion kind. When
 			this field is present, it will override the