docs: update batch.md to warn about automatic preparation

[piodul]

Add warnings about the fact that Session::batch now automatically prepares simple statements with non-empty value lists. Update the first example not to use such statements in order not to promote suboptimal usage of the API.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.