Misc updates to testing.md

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/docs/architecture/psa-migration/testing.md b/docs/architecture/psa-migration/testing.md
index 754686b..6ed294a 100644
--- a/docs/architecture/psa-migration/testing.md
+++ b/docs/architecture/psa-migration/testing.md
@@ -9,9 +9,11 @@
 ----------------------
 
 There needs to be at least one build in `all.sh` that enables
-`MBEDTLS_USE_PSA_CRYPTO` and runs the full battery of tests. Currently that's
+`MBEDTLS_USE_PSA_CRYPTO` and runs the full battery of tests; currently that's
 ensured by the fact that `scripts/config.py full` enables
-`MBEDTLS_USE_PSA_CRYPTO`.
+`MBEDTLS_USE_PSA_CRYPTO`. There needs to be at least one build with
+`MBEDTLS_USE_PSA_CRYPTO` disabled (as long as it's optional); currently that's
+ensured by the fact that it's disabled in the default config.
 
 Generally, code review is enough to ensure that PSA APIs are indeed used where
 they should be when `MBEDTLS_USE_PSA_CRYPTO` is enabled.
@@ -56,8 +58,9 @@
     (We should have something similar for
     `mbedtls_x509write_crt_set_issuer_key()`.)
 
-For some APIs, for example with `mbedtls_ssl_conf_psk_opaque()`, unit testing
-does not make sense, so we only have integration testing.
+For some APIs, for example with `mbedtls_ssl_conf_psk_opaque()`, testing in
+`test_suite_ssl` was historicaly not possible, so we only have testing in
+`ssl-opt.sh`.
 
 New APIs meant for internal use
 -------------------------------
@@ -89,7 +92,8 @@
 
 Changes in this category rarely require specific testing, as everything should
 be already be covered by running the existing tests in a build with
-`MBEDTLS_USE_PSA_CRYPTO` enabled.
+`MBEDTLS_USE_PSA_CRYPTO` enabled; however we need to make sure the existing
+test have sufficient coveraged, and improve them if necessary.
 
 However, if additional logic is involved, or there are run-time decisions about
 whether to use the PSA or legacy code paths, specific tests might be in order.