patch 8.0.1038: strike-through text not supported

Problem:    Strike-through text not supported.
Solution:   Add support for the "strikethrough" attribute. (Christian
            Brabandt, Ken Takata)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 0018eaa..859fbe8 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -4105,6 +4105,7 @@
 		s	standout	(termcap entry "so" and "se")
 		u	underline	(termcap entry "us" and "ue")
 		c	undercurl	(termcap entry "Cs" and "Ce")
+		t	strikethrough	(termcap entry "Ts" and "Te")
 		n	no highlighting
 		-	no highlighting
 		:	use a highlight group