Add sentence that was in 3.0 and is in TF-PSA-Crypto 1.0

Signed-off-by: Gilles Peskine <[email protected]>

Author: gilles-peskine-arm

docs/4.0-migration-guide.md

@@ -4,6 +4,8 @@ This guide details the steps required to migrate from Mbed TLS version 3.x to Mb
 
 Here's the list of breaking changes; each entry should help you answer these two questions: (1) am I affected? (2) if yes, what's my migration path?
 
+The changes are detailed below. Here is a summary of the main points:
+
 - Mbed TLS has been split between two products: TF-PSA-Crypto for cryptography, and Mbed TLS for X.509 and (D)TLS.
 - CMake is now the only supported build system.
 - The cryptography API is now mostly the PSA API: most legacy cryptography APIs have been removed. This has led to adaptations in some X.509 and TLS APIs, notably because the library always uses the PSA random generator.