Improve X509_ALGOR_new(3) documentation

Improve X509_ALGOR_new(3) documentation

The previous wording was misleading since the result of X509_ALGOR_new()
is not actually an empty X509_ALGOR object. Rather, it contains the
undefined ASN1_OBJECT returned by OBJ_nid2obj(NID_undef). Therefore using
X509_ALGOR_get0(3) for error checking X509_ALGOR_set_md() is not trivial.

So: change the initial paragraph into a general intro referring to the
OpenSSL API needed to interface with X509_ALGOR and write a new paragraph
documenting X509_ALGOR_new(3) and drop the incorrect suggestion of an error
check. Notably there's now a reference to the OBJ_nid2obj() family without
which one cannot really use X509_ALGOR_* for anything at all.

With and ok schwarze