docs(client): document Drop behavior for Connection types (#4064)

Explain what happens when Connection is dropped: the underlying IO
resource is closed and in-flight requests are interrupted. Suggest
polling to completion for graceful shutdown.

Closes #3192

Co-authored-by: lihan3238 <lihan3238@users.noreply.github.com>

Author: lihan3238

src/client/conn/http1.rs

@@ -50,6 +50,13 @@ pub struct Parts<T> {
 /// can process incoming and outgoing messages, notice hangups, and the like.
 ///
 /// Instances of this type are typically created via the [`handshake`] function
+///
+/// # Drop behavior
+///
+/// Dropping the `Connection` will close the underlying IO resource.
+/// Any in-flight requests that have not received a response will be
+/// interrupted. If graceful shutdown is desired, poll the connection
+/// until it completes instead of dropping.
 #[must_use = "futures do nothing unless polled"]
 pub struct Connection<T, B>
 where

src/client/conn/http2.rs

@@ -39,6 +39,13 @@ impl<B> Clone for SendRequest<B> {
 /// can process incoming and outgoing messages, notice hangups, and the like.
 ///
 /// Instances of this type are typically created via the [`handshake`] function
+///
+/// # Drop behavior
+///
+/// Dropping the `Connection` will close the underlying IO resource.
+/// Any in-flight requests that have not received a response will be
+/// interrupted. If graceful shutdown is desired, poll the connection
+/// until it completes instead of dropping.
 #[must_use = "futures do nothing unless polled"]
 pub struct Connection<T, B, E>
 where