patch 9.0.1397: highlight for popupmenu kind and extra cannot be set

Problem:    Highlight for popupmenu kind and extra cannot be set.
Solution:   Add PmenuKind, PmenuKindSel, PmenuExtra and PmenuExtraSel
            highlight groups and use them. (Gianmaria Bajo, closes #12114)
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index d624e4d..93c56a2 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -5452,6 +5452,14 @@
 Pmenu		Popup menu: Normal item.
 							*hl-PmenuSel*
 PmenuSel	Popup menu: Selected item.
+							*hl-PmenuKind*
+PmenuKind	Popup menu: Normal item "kind".
+							*hl-PmenuKindSel*
+PmenuKindSel	Popup menu: Selected item "kind".
+							*hl-PmenuExtra*
+PmenuExtra	Popup menu: Normal item "extra text".
+							*hl-PmenuExtraSel*
+PmenuExtraSel	Popup menu: Selected item "extra text".
 							*hl-PmenuSbar*
 PmenuSbar	Popup menu: Scrollbar.
 							*hl-PmenuThumb*