Function render 

pub fn render(
    cap: &Capability,
    hash_kind: Kind,
    agent: &str,
    offered: Offered,
    want_policy: WantPolicy,
) -> Option<String>

Render one capability’s wire spelling, or None if it is not on offer.

want_policy is a parameter and not a default because the advertisement genuinely depends on it, and a default would be the drift this whole table exists to prevent: the two allow-*-sha1-in-want rows are promises about what check_wants will serve, and a server that renders them from anything other than the policy it judges by is advertising a different server.