Improve description of struct-fields GUI test

Author: GuillaumeGomez

src/test/rustdoc-gui/struct-fields.goml

@@ -1,5 +1,5 @@
+// This test ensures that each field is on its own line (In other words, they have display: block).
 goto: "file://" + |DOC_PATH| + "/test_docs/struct.StructWithPublicUndocumentedFields.html"
 
-// Both fields must be on their own line. In other words, they have display: block.
 store-property: (first_top, "//*[@id='structfield.first']", "offsetTop")
 assert-property-false: ("//*[@id='structfield.second']", { "offsetTop": |first_top| })