docs(http/retry): document ReplayBody::poll_*() panics

[cratelyn]

a ReplayBody<B> demands a particular contract of its users: one must not poll two bodies in parallel. this is currently mentioned as an implicit assumption on an internal helper function, acquire_state<'a>, here:

https://github.com/linkerd/linkerd2-proxy/blob/main/linkerd/http/retry/src/replay.rs#L127-L131

    /// # Panics
    ///
    /// This panics if another clone has currently acquired the state, based on
    /// the assumption that a retry body will not be polled until the previous
    /// request has been dropped.
    fn acquire_state<'a>(
        state: &'a mut Option<BodyState<B>>,
        shared: &Mutex<Option<BodyState<B>>>,
    ) -> &'a mut BodyState<B> {
        state.get_or_insert_with(|| shared.lock().take().expect("missing body state"))
    }

this commit echoes this sentiment on the relevant public-facing parts of this middleware: its Body trait methods.