Update psa-limitations.md and add summary

Just reflecting recent/on-going work.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

docs/architecture/psa-migration/psa-limitations.md

@@ -11,6 +11,14 @@ is, of course, to actually do the migration work.
 Limitations relevant for G1 (performing crypto operations)
 ==========================================================
 
+Executive summary
+-----------------
+
+- Restartable/interruptible ECC operations: in progress (mid-2024).
+- Arbitrary parameters for FFDH: will be dropped in 4.0.
+- RSA-PSS parameters: already implemented safe though arguably non-compliant
+  solution in Mbed TLS 3.4, no complaints so far.
+
 Restartable (aka interruptible) ECC operations
 ----------------------------------------------
 
@@ -27,20 +35,9 @@ both enabled, some operations that should be restartable are not (ECDH in TLS
 operations that should use PSA do not (signature generation & verification) as
 they use the legacy API instead, in order to get restartable behaviour.
 
-Things that are in the API but not implemented yet
---------------------------------------------------
-
-PSA Crypto has an API for FFDH, but it's not implemented in Mbed TLS yet.
-(Regarding FFDH, see the next section as well.) See issue [3261][ffdh] on
-github.
-
-[ffdh]: https://github.com/Mbed-TLS/mbedtls/issues/3261
-
 Arbitrary parameters for FFDH
 -----------------------------
 
-(See also the first paragraph in the previous section.)
-
 Currently, the PSA Crypto API can only perform FFDH with a limited set of
 well-known parameters (some of them defined in the spec, but implementations
 are free to extend that set).