add CongestionControl::BlockFirst documentation

include/zenoh/api/enums.hxx

@@ -57,11 +57,16 @@ typedef ::z_reliability_t Reliability;
 
 ///  Congestion control values.
 ///
-///   Values:
+///  Values:
 ///    - **Z_CONGESTION_CONTROL_BLOCK**: Defines congestion control as "block". Messages are not dropped in case of
 ///       congestion control
 ///    - **Z_CONGESTION_CONTROL_DROP**: Defines congestion control as "drop". Messages are dropped in case of
-///    congestion control
+///      congestion control
+///    - **Z_CONGESTION_CONTROL_BLOCK_FIRST**: Defines congestion control as "block first". Messages except the first
+///      one are dropped in case of congestion control
+///  @warning **Z_CONGESTION_CONTROL_BLOCK_FIRST** has been marked as unstable: it works as advertised, but it may be
+///      changed in a future release.
+///  @note **Z_CONGESTION_CONTROL_BLOCK_FIRST** is Zenoh-c only.
 ///
 typedef ::z_congestion_control_t CongestionControl;