Remove sentences in RETURN VALUES sections saying that functions with
void return types 'return no value'. This is obvious and therefore
unneccessary to mention.
We spare rewind(3)'s sentence because espie@ pointed out that it's a
warning - the function masks a potential error.
This commit also adds a sentence to X509_free clarifying that it's
NULL-safe. This bit was discussed with doug@.
ok martijn@, sentiment supported by schwarze@