chore: Fix docs warnings when docs built with --all-features

[gretchenfrage]

On current master (a63603772ee1bc98957cf86eb3a904ed2357ba36) with Rust 1.82 (rustc 1.82.0 (f6e511eec 2024-10-15), cargo 1.82.0 (8f40fc59f 2024-08-21)), although both running cargo doc and running cargo check --all-features works without warnings, running cargo doc --all-features emits the following warnings:

Warnings

warning: unresolved link to `crate::client::connect::dns`
   --> src/client/legacy/connect/http.rs:171:30
    |
171 |     /// Takes a [`Resolver`](crate::client::connect::dns#resolvers-are-services) to handle DNS lookups.
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `connect` in module `client`
    |
    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: unresolved link to `Connection`
  --> src/client/legacy/connect/capture.rs:18:86
   |
18 | /// When making a request with Hyper, the underlying connection must implement the [`Connection`] trait.
   |                                                                                      ^^^^^^^^^^ no item named `Connection` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `hyper_util::server::con::auto`
  --> src/server/conn/auto/upgrade.rs:15:7
   |
15 | /// [`hyper_util::server::con::auto`] wraps the inner `T` with a private type
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `hyper_util` in scope

warning: unresolved link to `serve_connection_with_upgrades`
   --> src/server/conn/auto/mod.rs:112:45
    |
112 |     /// Does not do anything if used with [`serve_connection_with_upgrades`]
    |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `serve_connection_with_upgrades` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `serve_connection_with_upgrades`
   --> src/server/conn/auto/mod.rs:122:45
    |
122 |     /// Does not do anything if used with [`serve_connection_with_upgrades`]
    |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `serve_connection_with_upgrades` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `hyper_util::server::conn::auto::upgrade::downcast`
   --> src/server/conn/auto/mod.rs:170:47
    |
170 |     /// with this crate, you'll need to use [`hyper_util::server::conn::auto::upgrade::downcast`]
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `hyper_util` in scope

warning: redundant explicit link target
 --> src/client/legacy/connect/dns.rs:5:25
  |
5 | //! - A [`GaiResolver`](GaiResolver) that is the default resolver for the
  |          -------------  ^^^^^^^^^^^ explicit target is redundant
  |          |
  |          because label contains path that resolves to same destination
  |
  = note: when a link's destination is not specified,
          the label is used to resolve intra-doc links
  = note: `#[warn(rustdoc::redundant_explicit_links)]` on by default
help: remove explicit link target
  |
5 | //! - A [`GaiResolver`] that is the default resolver for the
  |         ~~~~~~~~~~~~~~~

warning: `hyper-util` (lib doc) generated 7 warnings

This PR fixes all such warnings.

[seanmonstar]

<3