lib: whitespace around Userdata invariants list #429

This fixes a nightly clippy finding: ``` error: doc list item missing indentation --> src/lib.rs:72:5 | 72 | /// If any of these invariants fails, try_drop will return an error. | ^ | = help: if this is supposed to be its own paragraph, add a blank line = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_lazy_continuation ```