[docs] update security-related limitations since 1.15.0 release

Since Kudu 1.15.0:
  * TLSv1.3 is supported for Kudu RPC
  * Kudu server Kerberos principals are configurable

Change-Id: Ibe05ca6ba178671f11bb33a7df85a23bb1c380b1
Reviewed-on: http://gerrit.cloudera.org:8080/17823
Tested-by: Kudu Jenkins
Reviewed-by: Bankim Bhavsar <bankim@cloudera.com>
2 files changed