Update spm.md

Author: danny4478

docs/porting/psa/spm.md

@@ -58,7 +58,7 @@ These is demonstrated in the example below:
 
 #### Memory layout
 
-Typically, PSA platforms share the same RAM and flash between secure and nonsecure cores. To provide PSA isolation level 1 or higher, you need to partition both RAM and flash in a way the following image describes:
+Typically, PSA platforms share the same RAM and flash between secure and nonsecure cores. To provide PSA isolation level 1 or higher, you need to partition both RAM and flash to secure and nonsecure parts, in a way the following image describes:
 
 ```text
                                  RAM
@@ -77,6 +77,8 @@ Typically, PSA platforms share the same RAM and flash between secure and nonsecu
 
 To achieve RAM and flash partitioning, you must add start and size values to a target configuration in `targets.json` as in the example above.
 
+Note that for isolation levels higher than 1, on top of the partitioning between secure and nonsecure parts, secure flash and RAM must have an inner level of partitioning, creating sections per secure partition.
+
 ### Linker scripts
 
 Linker scripts must include `MBED_ROM_START`, `MBED_ROM_SIZE`, `MBED_RAM_START` and `MBED_RAM_START` macros for defining memory regions. You can define a shared memory region by reserving RAM space for shared memory use. The shared memory location is target specific and depends on the memory protection scheme applied.