Reference mbedtls_f_rng_t in public documentation

In public functions that take `f_rng, p_rng` callbacks, link to the
documentation of the callback which is attached to the type name
`mbedtls_f_rng_t`.

Resolves #5868.

```
grep -l -w 'f_rng)' include | xargs perl -i -pe 's/\Qint (*f_rng)(void *, unsigned char *, size_t)\E/mbedtls_f_rng_t *f_rng/g'
```
and include `platform_util.h` where needed.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ssl_ticket.h b/include/mbedtls/ssl_ticket.h
index 2ee1400..bf3bc76 100644
--- a/include/mbedtls/ssl_ticket.h
+++ b/include/mbedtls/ssl_ticket.h
@@ -122,7 +122,7 @@
  *                  or a specific MBEDTLS_ERR_XXX error code
  */
 int mbedtls_ssl_ticket_setup(mbedtls_ssl_ticket_context *ctx,
-                             int (*f_rng)(void *, unsigned char *, size_t), void *p_rng,
+                             mbedtls_f_rng_t *f_rng, void *p_rng,
                              mbedtls_cipher_type_t cipher,
                              uint32_t lifetime);