PSA PAKE: add warning about stability

We are not confident about the stability of the PAKE interface (it is
just a proposal, not part of the standard yet).  So we should explicitly
document it as experimental, subject to change.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index 615b651..4478f3a 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -937,6 +937,14 @@
 /** @} */
 
 /** \defgroup pake Password-authenticated key exchange (PAKE)
+ *
+ * This is a proposed PAKE interface for the PSA Crypto API. It is not part of
+ * the official PSA Crypto API yet.
+ *
+ * \note The content of this section is not part of the stable API and ABI
+ *       of Mbed Crypto and may change arbitrarily from version to version.
+ *       Same holds for the corresponding macros #PSA_ALG_CATEGORY_PAKE and
+ *       #PSA_ALG_JPAKE.
  * @{
  */