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/ecjpake.h b/include/mbedtls/ecjpake.h
index c2148a2..7da8cb4 100644
--- a/include/mbedtls/ecjpake.h
+++ b/include/mbedtls/ecjpake.h
@@ -162,7 +162,7 @@
  */
 int mbedtls_ecjpake_write_round_one(mbedtls_ecjpake_context *ctx,
                                     unsigned char *buf, size_t len, size_t *olen,
-                                    int (*f_rng)(void *, unsigned char *, size_t),
+                                    mbedtls_f_rng_t *f_rng,
                                     void *p_rng);
 
 /**
@@ -203,7 +203,7 @@
  */
 int mbedtls_ecjpake_write_round_two(mbedtls_ecjpake_context *ctx,
                                     unsigned char *buf, size_t len, size_t *olen,
-                                    int (*f_rng)(void *, unsigned char *, size_t),
+                                    mbedtls_f_rng_t *f_rng,
                                     void *p_rng);
 
 /**
@@ -243,7 +243,7 @@
  */
 int mbedtls_ecjpake_derive_secret(mbedtls_ecjpake_context *ctx,
                                   unsigned char *buf, size_t len, size_t *olen,
-                                  int (*f_rng)(void *, unsigned char *, size_t),
+                                  mbedtls_f_rng_t *f_rng,
                                   void *p_rng);
 
 /**
@@ -266,7 +266,7 @@
  */
 int mbedtls_ecjpake_write_shared_key(mbedtls_ecjpake_context *ctx,
                                      unsigned char *buf, size_t len, size_t *olen,
-                                     int (*f_rng)(void *, unsigned char *, size_t),
+                                     mbedtls_f_rng_t *f_rng,
                                      void *p_rng);
 
 /**