Improve ChangeLog and migration guide entries

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/docs/3.0-migration-guide.d/mandatory-rng-param.md b/docs/3.0-migration-guide.d/mandatory-rng-param.md
index 3cbc356..f6aba08 100644
--- a/docs/3.0-migration-guide.d/mandatory-rng-param.md
+++ b/docs/3.0-migration-guide.d/mandatory-rng-param.md
@@ -5,16 +5,20 @@
 parameter with `NULL` as the value of this argument; this is no longer
 supported.
 
-The changed functions are: the X.509 CRT and CSR writing functions; the PK
-sign and decrypt function; the RSA encrypt, decrypt, sign and private
-functions; the functions in DHM and ECDH that compute the share secret; the
-scalar multiplication functions in ECP.
+The changed functions are: the X.509 CRT and CSR writing functions; the PK and
+RSA sign and decrypt functions; `mbedtls_rsa_private()`; the functions in DHM
+and ECDH that compute the shared secret; the scalar multiplication functions in
+ECP.
 
 You now need to pass a properly seeded, cryptographically secure RNG to all
 functions that accept a `f_rng` parameter. It is of course still possible to
 pass `NULL` as the context pointer `p_rng` if your RNG function doesn't need a
 context.
 
+Alternative implementations of a module (enabled with the `MBEDTLS_module_ALT`
+configuration options) may have their own internal and are free to ignore the
+`f_rng` argument but must allow users to pass one anyway.
+
 Some functions gained an RNG parameter
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