Abstractify example in design exploration

Since this is just an example, remove specific-sounding references to
mbedtls_psa_core_poison_memory() and replace with more abstract and
generic-sounding memory_poison_hook() and memory_unpoison_hook().

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/docs/architecture/psa-shared-memory.md b/docs/architecture/psa-shared-memory.md
index 65864fb..ffdb018 100644
--- a/docs/architecture/psa-shared-memory.md
+++ b/docs/architecture/psa-shared-memory.md
@@ -301,14 +301,14 @@
 ```c
 static void copy_to_user(void *copy_buffer, void *const input_buffer, size_t length) {
 #if defined(MBEDTLS_TEST_HOOKS)
-    if (mbedtls_psa_core_poison_memory != NULL) {
-        mbedtls_psa_core_poison_memory(copy_buffer, length, 0);
+    if (memory_poison_hook != NULL) {
+        memory_poison_hook(copy_buffer, length);
     }
 #endif
     memcpy(copy_buffer, input_buffer, length);
 #if defined(MBEDTLS_TEST_HOOKS)
-    if (mbedtls_psa_core_poison_memory != NULL) {
-        mbedtls_psa_core_poison_memory(copy_buffer, length, 1);
+    if (memory_unpoison_hook != NULL) {
+        memory_unpoison_hook(copy_buffer, length);
     }
 #endif
 }