d2i_X509.pod: clarify usage of the 'pp' function parameter

The 'pp' function parameters of d2i_TYPE() and i2d_TYPE() are referenced
in the DESCRIPTION section as 'in' resp. 'out'. This commit renames the
references to 'ppin' resp. 'ppout' and adds an explaining sentence.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5365)