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/eval.txt b/runtime/doc/eval.txt
index e8bf16e..aea1cce 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -7723,6 +7723,7 @@
 		"standout"	"1" if standout
 		"underline"	"1" if underlined
 		"undercurl"	"1" if undercurled
+		"strike"	"1" if strikethrough
 
 		Example (echoes the color of the syntax item under the
 		cursor): >