Improve documentation of truncated HMAC fallback option
/**
* \def MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
*
- * Fallback to old, non-conforming implementation of the truncated
- * HMAC extension which also truncates the HMAC key.
- *
- * \warning This should only be enabled temporarily when (1) the use of
- * truncated HMAC is essential in order to save bandwidth, and
- * (2) the peer is an Mbed TLS stack that doesn't use the fixed
- * implementation yet (version number <= 2.6.0).
+ * Fallback to old (pre-2.7), non-conforming implementation of the truncated
+ * HMAC extension which also truncates the HMAC key. Note that this option is
+ * only meant for a transitory upgrade period and is likely to be removed in
+ * a future version of the library.
+ *
+ * \warning The old implementation is non-compliant and has a security weakness
+ * (2^80 brute force attack on the HMAC key used for a single,
+ * uninterrupted connection). This should only be enabled temporarily
+ * when (1) the use of truncated HMAC is essential in order to save
+ * bandwidth, and (2) the peer is an Mbed TLS stack that doesn't use
+ * the fixed implementation yet (pre-2.7).
*
* \deprecated This option is deprecated and will likely be removed in a
* future version of Mbed TLS.
