Document limits on sharing transactions across threads

[x0Lazarus]

Transaction state belongs to the filesystem instance. When two threads share that instance and overlap transactions, one transaction can interfere with the other's pending writes. The transaction documentation currently has no warning about this.

Add the warning and explain how to use separate instances with skip_instance_cache=True. This follows the maintainer's recommendation in #180 to document the limitation. The changelog entry records the clarification; there is no runtime change.

Checked locally on Windows with Python 3.12:

• A temporary-file example reproduces interference with a shared instance and preserves the other thread's write with separate instances.
• Eight existing local transaction tests pass. The cross-device test is skipped on Windows by its existing marker.
• The changed documentation sections and Dev changelog build with warnings treated as errors in a minimal Sphinx project. The generated HTML contains the warning and a working link to Instance Caching. Codespell and git diff --check pass.

The full documentation site, cloud backends, downstream projects and other OS/Python combinations were not tested.

Refs #180.