Add docs on non-stability of internal function use (#3952)

(cherry picked from commit 717fbc4)
Signed-off-by: Hadrien Croubois <hadrien.croubois@gmail.com>

docs/modules/ROOT/pages/extending-contracts.adoc

@@ -126,4 +126,6 @@ That's it! Enjoy simpler code using hooks!
 
 The maintainers of OpenZeppelin Contracts are mainly concerned with the correctness and security of the code as published in the library, and the combinations of base contracts with the official extensions from the library.
 
-Custom overrides, and those of hooks in particular, may break some important assumptions and introduce vulnerabilities in otherwise secure code. While we try to ensure the contracts remain secure in the face of a wide range of potential customizations, this is done in a best-effort manner. Similarly, while we try to document all important assumptions, this should not be relied upon. Custom overrides should be carefully reviewed and checked against the source code of the contract they are customizing so as to fully understand their impact and guarantee their security.
+Custom overrides, and those of hooks in particular, may break some important assumptions and introduce vulnerabilities in otherwise secure code. While we try to ensure the contracts remain secure in the face of a wide range of potential customizations, this is done in a best-effort manner. While we try to document all important assumptions, this should not be relied upon. Custom overrides should be carefully reviewed and checked against the source code of the contract they are customizing so as to fully understand their impact and guarantee their security.
+
+The way functions interact internally should not be assumed to stay stable across releases of the library. For example, a function that is used in one context in a particular release may not be used in the same context in the next release. Contracts that override functions should revalidate their assumptions when updating the version of OpenZeppelin Contracts they are built on.