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_cookie.h b/include/mbedtls/ssl_cookie.h
index 71c258e..0f211e6 100644
--- a/include/mbedtls/ssl_cookie.h
+++ b/include/mbedtls/ssl_cookie.h
@@ -70,7 +70,7 @@
  * \brief          Setup cookie context (generate keys)
  */
 int mbedtls_ssl_cookie_setup(mbedtls_ssl_cookie_ctx *ctx,
-                             int (*f_rng)(void *, unsigned char *, size_t),
+                             mbedtls_f_rng_t *f_rng,
                              void *p_rng);
 
 /**