dbus_proxy macro should generate docs

Currently if a library adds a #[deny(missing_docs)] you will notice that it complains that dbus_proxy doesn't provide any documentation of the generated functions.

See

error: missing documentation for an associated function
   --> src/flatpak/mod.rs:122:1
    |
122 | / #[dbus_proxy(
123 | |     interface = "org.freedesktop.portal.Flatpak",
124 | |     default_service = "org.freedesktop.portal.Flatpak",
125 | |     default_path = "/org/freedesktop/portal/Flatpak"
126 | | )]
    | |__^
    |
    = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

The code looks like this

#[dbus_proxy(
    interface = "org.freedesktop.portal.Flatpak",
    default_service = "org.freedesktop.portal.Flatpak",
    default_path = "/org/freedesktop/portal/Flatpak"
)]
/// The interface exposes some interactions with Flatpak on the host to the sandbox.
/// For example, it allows you to restart the applications or start a more sandboxed instance.
trait Flatpak {

}

Per https://stackoverflow.com/a/41362344/5606146 the macro should either generates the docs itself or let the user do that