clarify why we're suggesting removing semicolon after braced items

Previously (issue #46186, pull-request #46258), a suggestion was added
to remove the semicolon after we fail to parse an item, but issue #51603
complains that it's still insufficiently obvious why. Let's add a note.

Resolves #51603.

src/test/ui/issue-46186.stderr

@@ -3,6 +3,8 @@ error: expected item, found `;`
    |
 LL | }; //~ ERROR expected item, found `;`
    |  ^ help: consider removing this semicolon
+   |
+   = help: braced struct declarations are not followed by a semicolon
 
 error: aborting due to previous error