Clarify return value of pk_check_pair()
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 3a35afb..d70e546 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -503,7 +503,11 @@
* \param pub Context holding a public key.
* \param prv Context holding a private (and public) key.
*
- * \return 0 on success or MBEDTLS_ERR_PK_BAD_INPUT_DATA
+ * \return \c 0 on success (keys were checked and match each other).
+ * \return #MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE if the keys could not
+ * be checked - in that case they may or may not match.
+ * \return #MBEDTLS_ERR_PK_BAD_INPUT_DATA if a context is invalid.
+ * \return Another non-zero value if the keys do not match.
*/
int mbedtls_pk_check_pair( const mbedtls_pk_context *pub, const mbedtls_pk_context *prv );