DOC Document compatibility across sklearn versions (#290)

* [skip ci] Document compatibility across versions

Resolves #270

Add a section to the persistence docs that cautions about using skops to
load sklearn models saved with one version while using another version.
Refers to sklearn docs on that issue.

* Update docs/persistence.rst

Co-authored-by: Adrin Jalali <adrin.jalali@gmail.com>

* Update docs/persistence.rst

Co-authored-by: Adrin Jalali <adrin.jalali@gmail.com>

* wrap lines

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Co-authored-by: Adrin Jalali <adrin.jalali@gmail.com>

docs/persistence.rst

@@ -160,8 +160,31 @@ Therefore, you should only load a skops file containing a model of any of those
 libraries if you trust them to be secure. It's not a perfect solution, but it's
 still better than trusting pickle files, which anyone can tamper with easily.
 
+Compatibility across sklearn versions
+-------------------------------------
+
+Using skops to load a model saved in one sklearn version and loading it with
+another sklearn version is not recommended, because the behavior of the model
+may change across versions. In some cases loading the model in a different
+version might not be possible due to internal changes in scikit-learn. Such
+changes don't happen very often, but they can happen, thus you should be
+cautious. To replicate a model trained with one sklearn version using a
+different sklearn version, it is advised to retrain the model on the same data
+using the same training process.
+
+The potential compatibility issue between sklearn versions is not skops
+specific. It is general sklearn behavior which skops cannot avoid. According to
+the sklearn `docs on model persistence
+<https://scikit-learn.org/stable/model_persistence.html#security-maintainability-limitations>`_:
+
+    While models saved using one version of scikit-learn might load in other
+    versions, this is entirely unsupported and inadvisable. It should also be
+    kept in mind that operations performed on such data could give different and
+    unexpected results.
+
 Roadmap
 -------
+
 There needs to be more testing to harden the loader and make sure we don't run
 arbitrary code when it's not intended. However, the safety mechanisms already
 in place should prevent most cases of abuse.