PSA_ALG_HKDF: add salt processing warning
* You may pass #PSA_KEY_DERIVATION_INPUT_INFO at any time after steup and before
* starting to generate output.
*
+ * \warning HKDF processes the salt as follows: first hash it with hash_alg
+ * if the salt is longer than the block size of the hash algorithm; then
+ * pad with null bytes up to the block size. As a result, it is possible
+ * for distinct salt inputs to result in the same outputs. To ensure
+ * unique outputs, it is recommended to use a fixed length for salt values.
+ *
* \param hash_alg A hash algorithm (\c PSA_ALG_XXX value such that
* #PSA_ALG_IS_HASH(\p hash_alg) is true).
*
