Doctest column name reference (#918)

* Doctest column name reference
* Document rusqlite assumption on column name reference
And move doctest as a test.
* Document when columns metadata should be extracted.
* Rustfmt doc (wrap_comments)

src/session.rs

@@ -411,7 +411,8 @@ impl Drop for ChangesetIter<'_> {
 }
 
 /// `feature = "session"` An item passed to a conflict-handler by
-/// [`Connection::apply`](crate::Connection::apply), or an item generated by [`ChangesetIter::next`](ChangesetIter::next).
+/// [`Connection::apply`](crate::Connection::apply), or an item generated by
+/// [`ChangesetIter::next`](ChangesetIter::next).
 // TODO enum ? Delete, Insert, Update, ...
 pub struct ChangesetItem {
     it: *mut ffi::sqlite3_changeset_iter,