expand: fix minor diagnostics bug

The error mentions `///`, when it's actually `//!`:

error[E0658]: attributes on expressions are experimental
 --> test.rs:4:9
  |
4 |         //! wah
  |         ^^^^^^^
  |
  = note: see issue rust-lang#15701 <rust-lang#15701> for more information
  = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
  = help: `///` is for documentation comments. For a plain comment, use `//`.

compiler/rustc_expand/src/config.rs

@@ -387,7 +387,11 @@ impl<'a> StripUnconfigured<'a> {
             );
 
             if attr.is_doc_comment() {
-                err.help("`///` is for documentation comments. For a plain comment, use `//`.");
+                err.help(if attr.style == AttrStyle::Outer {
+                    "`///` is used for outer documentation comments; for a plain comment, use `//`."
+                } else {
+                    "`//!` is used for inner documentation comments; for a plain comment, use `//` by removing the `!` or inserting a space in between them: `// !`"
+                });
             }
 
             err.emit();

tests/ui/feature-gates/feature-gate-stmt_expr_attributes.stderr

@@ -17,7 +17,7 @@ LL |     /// foo
    = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
    = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: `///` is for documentation comments. For a plain comment, use `//`.
+   = help: `///` is used for outer documentation comments; for a plain comment, use `//`.
 
 error[E0658]: attributes on expressions are experimental
   --> $DIR/feature-gate-stmt_expr_attributes.rs:10:5
@@ -28,7 +28,7 @@ LL |     //! foo
    = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
    = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: `///` is for documentation comments. For a plain comment, use `//`.
+   = help: `//!` is used for inner documentation comments; for a plain comment, use `//` by removing the `!` or inserting a space in between them: `// !`
 
 error: aborting due to 3 previous errors
 

tests/ui/lint/unused/unused-doc-comments-edge-cases.stderr

@@ -13,7 +13,7 @@ LL |     /// useless doc comment
    = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
    = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: `///` is for documentation comments. For a plain comment, use `//`.
+   = help: `///` is used for outer documentation comments; for a plain comment, use `//`.
 
 error: unused doc comment
   --> $DIR/unused-doc-comments-edge-cases.rs:6:9