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/lms.h b/include/mbedtls/lms.h
index 95fce21..2f29791 100644
--- a/include/mbedtls/lms.h
+++ b/include/mbedtls/lms.h
@@ -364,7 +364,7 @@
 int mbedtls_lms_generate_private_key(mbedtls_lms_private_t *ctx,
                                      mbedtls_lms_algorithm_type_t type,
                                      mbedtls_lmots_algorithm_type_t otstype,
-                                     int (*f_rng)(void *, unsigned char *, size_t),
+                                     mbedtls_f_rng_t *f_rng,
                                      void *p_rng, const unsigned char *seed,
                                      size_t seed_size);
 
@@ -427,7 +427,7 @@
  * \return         A non-zero error code on failure.
  */
 int mbedtls_lms_sign(mbedtls_lms_private_t *ctx,
-                     int (*f_rng)(void *, unsigned char *, size_t),
+                     mbedtls_f_rng_t *f_rng,
                      void *p_rng, const unsigned char *msg,
                      unsigned int msg_size, unsigned char *sig, size_t sig_size,
                      size_t *sig_len);