rustdoc: add test for strikethrough with single tildes

Also merge the two almost identical strikethrough tests together and document what the test tests.

tests/rustdoc/test-strikethrough.rs

@@ -1,6 +1,13 @@
 #![crate_name = "foo"]
 
-// @has foo/fn.f.html
-// @has - //del "Y"
-/// ~~Y~~
+// Test that strikethrough works with single and double tildes and that it shows up on
+// the item's dedicated page as well as the parent module's summary of items.
+
+// @has foo/index.html //del 'strike'
+// @has foo/index.html //del 'through'
+
+// @has foo/fn.f.html //del 'strike'
+// @has foo/fn.f.html //del 'through'
+
+/// ~~strike~~ ~through~
 pub fn f() {}