Improve compiler version checking + docs + testing for armclang

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index f881105..4be2a21 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -2258,13 +2258,14 @@
  *          system, Armv8-A Cryptographic Extensions must be supported by
  *          the CPU when this option is enabled.
  *
- * \note    Minimum compiler versions for this feature are Clang 4.0,
- *          armclang 6.6, GCC 6.0 or MSVC 2019 version 16.11.2.
+ * \note    Minimum compiler versions for this feature are Clang 4.0;
+ *          armclang 6.6 when targeting aarch64, or 6.20 when targeting
+ *          Thumb or 32-bit Arm; GCC 6.0; or MSVC 2019 version 16.11.2.
  *
  * \note \c CFLAGS must be set to a minimum of \c -march=armv8-a+crypto for
  * armclang <= 6.9
  *
- * This module adds support for the AES Armv8-A Cryptographic Extensions on Aarch64 systems.
+ * This module adds support for the AES Armv8-A Cryptographic Extensions on Armv8 systems.
  */
 #define MBEDTLS_AESCE_C