DOC: Add a few previously documented functions

DOC: Add a few previously documented functions

d2i_X509_bio(), d2i_X509_fp(), i2d_X509_bio(), and i2d_X509_fp()
were documented in OpenSSL 1.0.2.  In a grand unification of the
documentation of (almost) all d2i and i2d functions, these were
dropped, most likely by mistake.

This simply adds them back.

Fixes #17091

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17094)

(cherry picked from commit 1aa96159b0711369de9bf45b20327414cdb6b991)