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 #15701 <#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 `//`.

Author: Xiretza

compiler/rustc_expand/messages.ftl

@@ -12,7 +12,8 @@ expand_attribute_single_word =
 
 expand_attributes_on_expressions_experimental =
     attributes on expressions are experimental
-    .help_outer_doc = `///` is for documentation comments. For a plain comment, use `//`.
+    .help_outer_doc = `///` is used for outer documentation comments; for a plain comment, use `//`.
+    .help_inner_doc = `//!` is used for inner documentation comments; for a plain comment, use `//` by removing the `!` or inserting a space in between them: `// !`
 
 expand_attributes_wrong_form =
     attribute must be of form: `attributes(foo, bar)`

compiler/rustc_expand/src/config.rs

@@ -387,7 +387,11 @@ impl<'a> StripUnconfigured<'a> {
             );
 
             if attr.is_doc_comment() {
-                err.help(crate::fluent_generated::expand_help_outer_doc);
+                err.help(if attr.style == AttrStyle::Outer {
+                    crate::fluent_generated::expand_help_outer_doc
+                } else {
+                    crate::fluent_generated::expand_help_inner_doc
+                });
             }
 
             err.emit();