docs/API: remove duplicate description for SRTO_KMPREANNOUNCE

Remove the duplicate description for SRTO_KMPREANNOUNCE

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

docs/API.md

@@ -584,27 +584,6 @@ might still be in flight, or packets that have to be retransmitted.
 | --------------------- | ----- | ------- | --------- | ------ | -------- | ------ |
 | `SRTO_KMPREANNOUNCE`  | 1.3.2 | pre     | `int32_t` | pkts   | 0x1000 | see below |
 
-- **[GET or SET]** - The interval (defined in packets) between when a new
-  Stream Encrypting Key (SEK) is sent and when switchover occurs. This value
-also applies to the subsequent interval between when switchover occurs and when
-the old SEK is decommissioned.
-
-At `SRTO_KMPREANNOUNCE` packets before switchover the new key is sent
-(repeatedly, if necessary, until it is confirmed by the receiver).
-
-At the switchover point (see `SRTO_KMREFRESHRATE`), the sender starts
-  encrypting and sending packets using the new key. The old key persists in
-  case it is needed to decrypt packets that were in the flight window, or
-  retransmitted packets.
-
-The old key is decommissioned at `SRTO_KMPREANNOUNCE` packets after switchover . 
-
-The allowed range for this value is between 1 and half of the current value
-of `SRTO_KMREFRESHRATE`. The minimum value should never be less than the
-flight window (i.e. the number of packets that have already left the sender but
-have not yet arrived at the receiver).
-
-
 - **[GET or SET]** - The interval (defined in packets) between when a new
   Stream Encrypting Key (SEK) is sent and when switchover occurs. This value
 also applies to the subsequent interval between when switchover occurs and when