docs: add rustdoc comments to all module options

This part of an upcoming effort to generate documentation from code.

Pushing this out before that stage so that the JSON schema is fully documented.

src/config/truncate.rs

@@ -20,13 +20,68 @@ impl From<EllipsizeMode> for GtkEllipsizeMode {
     }
 }
 
+/// Some modules provide options for truncating text.
+/// This is controlled using a common `TruncateMode` type,
+/// which is defined below.
+///
+/// The option can be configured in one of two modes.
+///
 #[derive(Debug, Deserialize, Clone, Copy)]
 #[serde(untagged)]
 pub enum TruncateMode {
+    /// Auto mode lets GTK decide when to ellipsize.
+    ///
+    /// To use this mode, set the truncate option to a string
+    /// declaring the location to truncate text from and place the ellipsis.
+    ///
+    /// # Example
+    ///
+    /// ```corn
+    /// { truncate = "start" }
+    /// ```
+    ///
+    /// **Valid options**: `start`, `middle`, `end`
+    /// <br>
+    /// **Default**: `null`
     Auto(EllipsizeMode),
+
+    /// Length mode defines a fixed point at which to ellipsize.
+    ///
+    /// Generally you will want to set only one of `length` or `max_length`,
+    /// but you can set both if required.
+    ///
+    /// # Example
+    ///
+    /// ```corn
+    /// {
+    ///     truncate.mode = "start"
+    ///     truncate.length = 50
+    ///     truncate.max_length = 70
+    /// }
+    /// ```
     Length {
+        /// The location to truncate text from and place the ellipsis.
+        /// **Valid options**: `start`, `middle`, `end`
+        /// <br>
+        /// **Default**: `null`
         mode: EllipsizeMode,
+
+        /// The fixed width (in characters) of the widget.
+        ///
+        /// The widget will be expanded to this width
+        /// if it would have otherwise been smaller.
+        ///
+        /// Leave unset to let GTK automatically handle.
+        ///
+        /// **Default**: `null`
         length: Option<i32>,
+
+        /// The maximum number of characters to show
+        /// before truncating.
+        ///
+        /// Leave unset to let GTK automatically handle.
+        ///
+        /// **Default**: `null`
         max_length: Option<i32>,
     },
 }