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.h b/include/mbedtls/ssl.h
index cc9da34..960200f 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -2260,7 +2260,7 @@
  * \param p_rng    RNG parameter
  */
 void mbedtls_ssl_conf_rng(mbedtls_ssl_config *conf,
-                          int (*f_rng)(void *, unsigned char *, size_t),
+                          mbedtls_f_rng_t *f_rng,
                           void *p_rng);
 
 /**