Correct documented return value for BIO_get_mem_data()

CLA: trivial
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9643)

(cherry picked from commit 797a5b7af9d9bbfbcbff4607c10ad5c5595ac785)

doc/man3/BIO_s_mem.pod

@@ -122,9 +122,12 @@ There should be an option to set the maximum size of a memory BIO.
 
 BIO_s_mem() and BIO_s_secmem() return a valid memory B<BIO_METHOD> structure.
 
-BIO_set_mem_eof_return(), BIO_get_mem_data(), BIO_set_mem_buf() and BIO_get_mem_ptr()
+BIO_set_mem_eof_return(), BIO_set_mem_buf() and BIO_get_mem_ptr()
 return 1 on success or a value which is less than or equal to 0 if an error occurred.
 
+BIO_get_mem_data() returns the total number of bytes available on success,
+0 if b is NULL, or a negative value in case of other errors.
+
 BIO_new_mem_buf() returns a valid B<BIO> structure on success or NULL on error.
 
 =head1 EXAMPLES