Unify d2i/i2d documentation.

Make d2i_X509 a generic d2i/i2d manpage.
Pull common stuff out of other d2i/i2d docs.
Update find-doc-nits to know about "generic" manpages.
Cleanup some overlap.
Fix up a bunch of other references.

Reviewed-by: Matt Caswell <matt@openssl.org>