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/x509_crt.h b/include/mbedtls/x509_crt.h
index 1ce0d23..6b96039 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -1176,7 +1176,7 @@
  * \note            \p f_rng is used for the signature operation.
  */
 int mbedtls_x509write_crt_der(mbedtls_x509write_cert *ctx, unsigned char *buf, size_t size,
-                              int (*f_rng)(void *, unsigned char *, size_t),
+                              mbedtls_f_rng_t *f_rng,
                               void *p_rng);
 
 #if defined(MBEDTLS_PEM_WRITE_C)
@@ -1194,7 +1194,7 @@
  * \note            \p f_rng is used for the signature operation.
  */
 int mbedtls_x509write_crt_pem(mbedtls_x509write_cert *ctx, unsigned char *buf, size_t size,
-                              int (*f_rng)(void *, unsigned char *, size_t),
+                              mbedtls_f_rng_t *f_rng,
                               void *p_rng);
 #endif /* MBEDTLS_PEM_WRITE_C */
 #endif /* MBEDTLS_X509_CRT_WRITE_C */