libsyntax: Explicit error message for sugared doc comments.

Display an explicit message about items missing after sugared doc
comment attributes. References #2789.
auto merge of #17218 : iliekturtles/rust/2789-invalid-doc-comment, r=huonw

Display an explicit message about items missing after sugared doc
comment attributes. References #2789.

 * I tried looking through `parser.rs` for an appropriate location for `expected_item_err` and ended up putting it just above the first use. Is there a better location?
 * Did I add enough test cases? Too many? Should I add more cases for the original error message?