Add missing 'RETURN VALUES' sections in doc

All missing sections are added.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4976)
diff --git a/doc/man3/RSA_generate_key.pod b/doc/man3/RSA_generate_key.pod
index ba07e6f..5050a8e 100644
--- a/doc/man3/RSA_generate_key.pod
+++ b/doc/man3/RSA_generate_key.pod
@@ -73,7 +73,7 @@
 The process is then repeated for prime q and other primes (if any)
 with B<BN_GENCB_call(cb, 3, i)> where B<i> indicates the i-th prime.
 
-=head1 RETURN VALUE
+=head1 RETURN VALUES
 
 RSA_generate_multi_prime_key() returns 1 on success or 0 on error.
 RSA_generate_key_ex() returns 1 on success or 0 on error.