Update docs/porting/psa/spm.md

Co-Authored-By: danny4478 <[email protected]>

Author: Alexander Zilberkant

docs/porting/psa/spm.md

@@ -187,7 +187,7 @@ define symbol __ICFEDIT_region_IROM1_end__   = (MBED_ROM_START + MBED_ROM_SIZE);
 
 ### Mailbox
 
-Mailbox is the SPM mechanism in charge of IPC, and is **relevant for multicore systems only**.
+Mailbox is the mechanism used to implement Inter Processor Communication and **only relevant for multicore systems**. Mailbox is used by SPM for communicating with secure partitions from nonsecure processing environment.
 
 #### Concepts