Adjust tests/ui/attr-bad-crate-attr.rs

- Move `tests/ui/attr-bad-crate-attr.rs` to `tests/ui/attributes/`.
- Briefly document test intent add link to relevant Reference docs.

Author: jieyouxu

Diff for: tests/ui/attr-bad-crate-attr.rs

@@ -0,0 +1,9 @@
+//! Check that we permit a crate-level inner attribute but reject a dangling outer attribute which
+//! does not have a following thing that it can target.
+//!
+//! See <https://doc.rust-lang.org/reference/attributes.html>.
+
+//@ error-pattern: expected item
+
+#![attr = "val"]
+#[attr = "val"] // Unterminated

Diff for: tests/ui/attributes/attr-bad-crate-attr.rs

@@ -1,5 +1,5 @@
 error: expected item after attributes
-  --> $DIR/attr-bad-crate-attr.rs:4:1
+  --> $DIR/attr-bad-crate-attr.rs:9:1
    |
 LL | #[attr = "val"] // Unterminated
    | ^^^^^^^^^^^^^^^