patch 8.0.0914: highlight attributes are always combined

Problem:    Highlight attributes are always combined.
Solution:   Add the 'nocombine' value to replace attributes instead of
            combining them. (scauligi, closes #1963)

runtime/doc/syntax.txt

@@ -4689,6 +4689,7 @@ the same syntax file on all terminals, and use the optimal highlighting.
 
 					*bold* *underline* *undercurl*
 					*inverse* *italic* *standout*
+					*nocombine*
 term={attr-list}			*attr-list* *highlight-term* *E418*
 	attr-list is a comma separated list (without spaces) of the
 	following items (in any order):
@@ -4699,6 +4700,7 @@ term={attr-list}			*attr-list* *highlight-term* *E418*
 		inverse		same as reverse
 		italic
 		standout
+		nocombine	override attributes instead of combining them
 		NONE		no attributes used (used to reset it)
 
 	Note that "bold" can be used here and by using a bold font.  They