Change error about unknown doc attributes to a warning

This prevents breakage across the ecosystem, since the error was just
introduced recently without first having a warning period.

src/test/rustdoc-ui/doc-attr.rs

@@ -1,5 +1,11 @@
 #![crate_type = "lib"]
-#![doc(as_ptr)] //~ ERROR
+#![deny(unused_attributes)]
+//~^ NOTE lint level is defined here
+#![doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 
-#[doc(as_ptr)] //~ ERROR
+#[doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 pub fn foo() {}

src/test/rustdoc-ui/doc-attr.stderr

@@ -1,14 +1,23 @@
 error: unknown `doc` attribute `as_ptr`
-  --> $DIR/doc-attr.rs:4:7
+  --> $DIR/doc-attr.rs:8:7
    |
 LL | #[doc(as_ptr)]
    |       ^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/doc-attr.rs:2:9
+   |
+LL | #![deny(unused_attributes)]
+   |         ^^^^^^^^^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unknown `doc` attribute `as_ptr`
-  --> $DIR/doc-attr.rs:2:8
+  --> $DIR/doc-attr.rs:4:8
    |
 LL | #![doc(as_ptr)]
    |        ^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: aborting due to 2 previous errors
 

src/test/ui/attributes/doc-attr.rs

@@ -1,5 +1,11 @@
 #![crate_type = "lib"]
-#![doc(as_ptr)] //~ ERROR
+#![deny(unused_attributes)]
+//~^ NOTE lint level is defined here
+#![doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 
-#[doc(as_ptr)] //~ ERROR
+#[doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 pub fn foo() {}

src/test/ui/attributes/doc-attr.stderr

@@ -1,14 +1,23 @@
 error: unknown `doc` attribute `as_ptr`
-  --> $DIR/doc-attr.rs:4:7
+  --> $DIR/doc-attr.rs:8:7
    |
 LL | #[doc(as_ptr)]
    |       ^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/doc-attr.rs:2:9
+   |
+LL | #![deny(unused_attributes)]
+   |         ^^^^^^^^^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unknown `doc` attribute `as_ptr`
-  --> $DIR/doc-attr.rs:2:8
+  --> $DIR/doc-attr.rs:4:8
    |
 LL | #![doc(as_ptr)]
    |        ^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: aborting due to 2 previous errors