Clarified mbedtls_ssl_conf_alpn_protocols() doc

Clarified the lifetime of the protos parameter passed in the
function mbedtls_ssl_conf_alpn_protocols().
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 00e1c6c..ad0e42c 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1762,8 +1762,11 @@
  * \brief          Set the supported Application Layer Protocols.
  *
  * \param conf     SSL configuration
- * \param protos   NULL-terminated list of supported protocols,
- *                 in decreasing preference order.
+ * \param protos   Pointer to a NULL-terminated list of supported protocols,
+ *                 in decreasing preference order. The pointer to the list is
+ *                 recorded by the library for later reference as required, so
+ *                 the lifetime of the table should be as long as the
+ *                 SSL configuration structure.
  *
  * \return         0 on success, or MBEDTLS_ERR_SSL_BAD_INPUT_DATA.
  */