patch 8.1.1276: cannot combine text properties with syntax highlighting

Problem:    Cannot combine text properties with syntax highlighting.
Solution:   Add the "combine" field to prop_type_add(). (closes #4343)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index bc18daa..e1ce00f 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -7218,6 +7218,9 @@
 				properties the one with the highest priority
 				will be used; negative values can be used, the
 				default priority is zero
+		   combine	when TRUE combine the highlight with any
+				syntax highlight; when omitted of FALSE syntax
+				highlight will not be used
 		   start_incl	when TRUE inserts at the start position will
 				be included in the text property
 		   end_incl	when TRUE inserts at the end position will be
diff --git a/runtime/doc/textprop.txt b/runtime/doc/textprop.txt
index 3752551..263c40e 100644
--- a/runtime/doc/textprop.txt
+++ b/runtime/doc/textprop.txt
@@ -1,4 +1,4 @@
-*textprop.txt*  For Vim version 8.1.  Last change: 2019 Jan 08
+*textprop.txt*  For Vim version 8.1.  Last change: 2019 May 05
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -57,6 +57,10 @@
 A text property normally has the name of a property type, which defines 
 how to highlight the text.  The property type can have these entries:
 	"highlight"	name of the highlight group to use
+	"combine"	when TRUE the text property highlighting is combined
+			with any syntax highligting, when omitted or FALSE the
+			text property highlighting replaces the syntax
+			highlighting
 	"priority"	when properties overlap, the one with the highest
 			priority will be used.
 	"start_incl"	when TRUE inserts at the start position will be