Update the documentation of RAND_cleanup

This function hasn't done anything since 2015, when
https://boringssl-review.googlesource.com/5792 made it a no-op.

Also move RAND_get_system_entropy_for_custom_prng under obscure
functions. It's usually not what you want.

Change-Id: Ica57552792acff3c5991e6db09c15112db09d133
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/61505
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>

include/openssl/rand.h

@@ -29,20 +29,6 @@ extern "C" {
 // event that sufficient random data can not be obtained, |abort| is called.
 OPENSSL_EXPORT int RAND_bytes(uint8_t *buf, size_t len);
 
-// RAND_get_system_entropy_for_custom_prng writes |len| bytes of random data
-// from a system entropy source to |buf|. The maximum length of entropy which
-// may be requested is 256 bytes. If more than 256 bytes of data is requested,
-// or if sufficient random data can not be obtained, |abort| is called.
-// |RAND_bytes| should normally be used instead of this function. This function
-// should only be used for seed values or where |malloc| should not be called
-// from BoringSSL. This function is not FIPS compliant.
-OPENSSL_EXPORT void RAND_get_system_entropy_for_custom_prng(uint8_t *buf,
-                                                            size_t len);
-
-// RAND_cleanup frees any resources used by the RNG. This is not safe if other
-// threads might still be calling |RAND_bytes|.
-OPENSSL_EXPORT void RAND_cleanup(void);
-
 
 // Obscure functions.
 
@@ -65,6 +51,16 @@ OPENSSL_EXPORT void RAND_enable_fork_unsafe_buffering(int fd);
 OPENSSL_EXPORT void RAND_reset_for_fuzzing(void);
 #endif
 
+// RAND_get_system_entropy_for_custom_prng writes |len| bytes of random data
+// from a system entropy source to |buf|. The maximum length of entropy which
+// may be requested is 256 bytes. If more than 256 bytes of data is requested,
+// or if sufficient random data can not be obtained, |abort| is called.
+// |RAND_bytes| should normally be used instead of this function. This function
+// should only be used for seed values or where |malloc| should not be called
+// from BoringSSL. This function is not FIPS compliant.
+OPENSSL_EXPORT void RAND_get_system_entropy_for_custom_prng(uint8_t *buf,
+                                                            size_t len);
+
 
 // Deprecated functions
 
@@ -93,6 +89,9 @@ OPENSSL_EXPORT int RAND_poll(void);
 // RAND_status returns one.
 OPENSSL_EXPORT int RAND_status(void);
 
+// RAND_cleanup does nothing.
+OPENSSL_EXPORT void RAND_cleanup(void);
+
 // rand_meth_st is typedefed to |RAND_METHOD| in base.h. It isn't used; it
 // exists only to be the return type of |RAND_SSLeay|. It's
 // external so that variables of this type can be initialized.