Add ChangeLog and migration guide entries

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

docs/3.0-migration-guide.d/cipher-delayed-output.md

@@ -0,0 +1,15 @@
+Calling `mbedtls_cipher_finish()` is mandatory for all multi-part operations
+----------------------------------------------------------------------------
+
+This only affect people who use the Cipher module to perform AEAD operations
+using the multi-part API.
+
+Previously, the documentation didn't state explicitly if it was OK to call
+`mbedtls_cipher_check_tag()` or `mbedtls_cipher_write_tag()` directly after
+the last call to `mbedtls_cipher_update()` - that is, without calling
+`mbedtls_cipher_finish()` in-between. If you code was missing that call,
+please add it and be prepared to get as much as 15 bytes of output.
+
+Currently the output is always 0 bytes, but it may be more when alternative
+implementations of the underlying primitives are in use, or with future
+versions of the library.