Remove warning on doc compilation

[bidoubiwa]

When running cargo doc a lot of warning are raised. For example

    |
373 |     /// Get [Document]s by batch.
    |              ^^^^^^^^ no item named `Document` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `Document`

These should be fixed