OSSL_PARAM.pod: document the mechanism to figure out buffer sizes
OSSL_PARAM.pod: document the mechanism to figure out buffer sizes

When requesting parameters, it's acceptable to make a first pass with
the |data| field of some parameters being NULL.  That can be used to
help the requestor to figure out dynamically what buffer size is
needed.  For variable size parameters, there's no other way to find
out.

Reviewed-by: Paul Dale <bc4a6b4cd7a8395c734c8eb5aef04a9c30188c85@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10025)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl