Add documentation for mbedtls_psa_hkdf_extract
#if defined(MBEDTLS_PSA_CRYPTO_C)
+/**
+ * \brief Take the input keying material \p ikm and extract from it a
+ * fixed-length pseudorandom key \p prk.
+ *
+ * \param alg A hash function.
+ * \param salt An optional salt value (a non-secret random value);
+ * if the salt is not provided, a string of all zeros
+ * of the length of the hash provided by \p alg is used
+ * as the salt.
+ * \param salt_len The length in bytes of the optional \p salt.
+ * \param ikm The input keying material.
+ * \param ikm_len The length in bytes of \p ikm.
+ * \param[out] prk A pseudorandom key of \p prk_len bytes.
+ * \param prk_size Size of the \p prk buffer in bytes.
+ * \param[out] prk_len On success, the length in bytes of the
+ * pseudorandom key in \p prk.
+ *
+ * \return 0 on success.
+ * \return #MBEDTLS_ERR_HKDF_BAD_INPUT_DATA when the parameters are invalid.
+ * \return An PSA_ERROR_* error for errors returned from the underlying
+ * PSA layer.
+ */
psa_status_t mbedtls_psa_hkdf_extract( psa_algorithm_t alg,
const unsigned char *salt, size_t salt_len,
const unsigned char *ikm, size_t ikm_len,
