Provide more in-depth migration guide after removal of null entropy.

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>

docs/3.0-migration-guide.d/remove-null-entropy.md

@@ -4,6 +4,8 @@ Remove the option to build the library without any entropy sources
 This does not affect users who use the default `config.h`, as this option was
 already off by default.
 
-If you were using the `MBEDTLS_TEST_NULL_ENTROPY` option, you can either use
-`MBEDTLS_ENTROPY_NV_SEED` or create a fake entropy function.
-
+If you were using the `MBEDTLS_TEST_NULL_ENTROPY` option and your platform
+doesn't have any entropy source, you should use `MBEDTLS_ENTROPY_NV_SEED`
+and make sure your device is provisioned with a strong random seed.
+Alternatively, for testing purposes only, you can create and register a fake
+entropy function.