Try to further clarify documentation
@@ -1388,6 +1388,10 @@ void mbedtls_ssl_set_bio( mbedtls_ssl_context *ssl,
* the maximum size datagram the DTLS layer will pass to the
* \c f_send() callback set using \c mbedtls_ssl_set_bio().
*
+ * \note The limit on datagram size is converted to a limit on
+ * record payload by subtracting the current overhead of
+ * encapsulation and encryption/authentication if any.
+ *
* \note This can be called at any point during the connection, for
* example when a PMTU estimate becomes available from other
* sources, such as lower (or higher) protocol layers.
@@ -1400,14 +1404,12 @@ void mbedtls_ssl_set_bio( mbedtls_ssl_context *ssl,
*
* \note If both a MTU and a maximum fragment length have been
* configured (or negotiated with the peer), the resulting
- * lower limit (after translating the MTU setting to a limit
- * on the record content length) is used.
+ * lower limit on record payload (see first note) is used.
*
* \note This can only be used to decrease the maximum size
- * of datagrams (hence records, as records cannot span
- * multiple datagrams) sent. It cannot be used to increase the
- * maximum size of records over the limit set by
- * #MBEDTLS_SSL_OUT_CONTENT_LEN.
+ * of datagrams (hence records, see first note) sent. It
+ * cannot be used to increase the maximum size of records over
+ * the limit set by #MBEDTLS_SSL_OUT_CONTENT_LEN.
*
* \note Values lower than the current record layer expansion will
* result in an error when trying to send data.
