Add comments to NULL func ptrs in bio_method_st

This commit adds comments to bio_method_st definitions where the
function pointers are defined as NULL. Most of the structs have comments
but some where missing and not all consitent.

CLA: trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4881)