Minimally document PKCS7_dataInit(3).

No comment when shown around among LibreSSL devs
except "very very strange function" from beck@
and "cannot say much about it" from tb@.

If needed, this can be

Minimally document PKCS7_dataInit(3).

No comment when shown around among LibreSSL devs
except "very very strange function" from beck@
and "cannot say much about it" from tb@.

If needed, this can be further polished in the tree,
review is still welcome.

show more ...

new manual page for PKCS7_set_content(3) and PKCS7_content_new(3);
OK beck@ tb@