docs(http/upgrade): document linkerd-http-upgrade

[cratelyn]

some aspects of linkerd-http-upgrade are incompatible with the 1.0 interface of the http crate (see: hyperium/http#395, linkerd/linkerd2#8733).

this new bound requiring that extensions must now be cloneable motivated me to read through this library's internals to gain a lucid understanding of how it works, in order to understand how to gracefully address this comment affixed to the linkerd_http_upgrade::upgrade::Http11Upgrade request/response extension:

// Note: this relies on their only having been 2 Inner clones, so don't
// implement `Clone` for this type. [sic]
pub struct Http11Upgrade {
    half: Half,
    inner: Arc<Inner>,
}

broadly, this library deals with some moderately arcane corners of the HTTP protocol family. the Upgrade header is not supported in HTTP/2, and was not yet introduced in HTTP/1.0, so it is a feature specific to HTTP/1.1. moreover, some behavior provided by this library falls into parts of the spec(s) that we MUST uphold, and isn't currently well documented.

this branch includes a sequence of commits adding documentation and additional comments linking to, and quoting, the relevant parts of RFC 9110. some links to RFC 7231, which was obsoleted by RFC 9110 since the original time of writing, are additionally updated.

some comments also did not accurately describe internal logic, or included typos, and are also updated.