docs: cli flag help and Hints field docstring for schedule

crates/app/cli.rs

@@ -166,7 +166,7 @@ pub struct SendCommand {
         short = 's',
         long,
         help = "Schedule",
-        long_help = "Schedule notification"
+        long_help = "Specifies the time to schedule the notification to be shown."
     )]
     schedule: String,
 }

crates/dbus/src/notification.rs

@@ -90,6 +90,8 @@ pub struct Hints {
     /// The localized display name will be used to annotate the icon for accessibility purposes.
     /// The icon name should be compliant with the Freedesktop.org Icon Naming Specification.
     pub action_icons: Option<bool>,
+
+    /// Specifies the time to schedule the notification to be shown.
     pub schedule: Option<String>,
 }