Misc cleanup points noticed in #1752

[gretchenfrage]

I'm creating this issue as a list of miscellaneous things that we're noticing should be cleaned up in the course of developing #1752 but which would not be in scope for that issue itself:

• quinn::Connecting has a #[must_use] annotation denoting that it is a future which is obsolete as Rust now does that automatically
• quinn::Connecting::local_ip is missing a comment stating that it will panic if called after poll has returned Ready like its remote_address method has
• quinn::Connecting::remote_address has a trailing punctuation on the first line of its doc comment, which is inconsistent with general project style
• quinn does not re-export quinn_proto::TransportError and other downstream-API-exposed types despite that being a public part of quinn's API via quinn's re-exporting of quinn_proto::ConnectionError