Add migration guide for removal of mbedtls_ssl_get_session_pointer()

Signed-off-by: Hanno Becker <hanno.becker@arm.com>

docs/3.0-migration-guide.d/remove-ssl-get-session_pointer.md

@@ -0,0 +1,23 @@
+Remove the SSL API mbedtls_ssl_get_session_pointer()
+-----------------------------------------------------------------
+
+This affects two classes of users:
+
+1. Users who manually inspect parts of the current session through
+   direct structure field access.
+
+2. Users of session resumption who query the current session
+   via `mbedtls_ssl_get_session_pointer()` prior to saving or exporting
+   it via `mbedtls_ssl_session_copy()` or `mbedtls_ssl_session_save()`,
+   respectively.
+
+Migration paths:
+
+1. Mbed TLS 3.0 does not offer a migration path for the usecase 1: Like many
+   other Mbed TLS structures, the structure of `mbedtls_ssl_session` is no
+   longer part of the public API in Mbed TLS 3.0, and direct structure field
+   access is no longer supported. Please see the corresponding migration guide.
+
+2. Users should replace calls to `mbedtls_ssl_get_session_pointer()` by
+   calls to `mbedtls_ssl_get_session()` as demonstrated in the example
+   program `programs/ssl/ssl_client2.c`.