Warning for uVisor

Author: Irit Arkin

docs/reference/api/security/security.md

@@ -5,6 +5,8 @@ Security on Arm Mbed OS is divided into two parts:
 - [Connection security through Arm Mbed TLS](/docs/development/reference/tls.html).
 - [Device security through Arm Mbed uVisor](/docs/development/reference/uvisor.html).
 
+<span class="warnings">**Warning**: uVisor is superseded by the Secure Partition Manager (SPM) defined in the ARM Platform Security Architecture (PSA). uVisor is deprecated as of Mbed OS 5.9, and being replaced by a native PSA-compliant implementation of SPM.</span>
+
 The sections cover working with these modules in the context of Mbed OS. For generic documentation for both modules, see:
 
 - [Mbed TLS full documentation](https://tls.mbed.org/).