Fix a couple nits in DEFINE_STACK_OF.pod

Only the 'new' variant of sk_TYPE_new_reserve() deals with
compression functions.

Mention both new 'reserve' APIs as being added in OpenSSL 1.1.1.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4591)