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/modules/cairo.rs

@@ -19,16 +19,33 @@ use tracing::{debug, error};
 
 #[derive(Debug, Clone, Deserialize)]
 pub struct CairoModule {
+    /// The path to the Lua script to load.
+    /// This can be absolute, or relative to the working directory.
+    ///
+    /// The script must contain the entry `draw` function.
+    ///
+    /// **Required**
     path: PathBuf,
 
+    /// The number of milliseconds between each draw call.
+    ///
+    /// **Default**: `200`
     #[serde(default = "default_frequency")]
     frequency: u64,
 
+    /// The canvas width in pixels.
+    ///
+    /// **Default**: `42`
     #[serde(default = "default_size")]
     width: u32,
+
+    /// The canvas height in pixels.
+    ///
+    /// **Default**: `42`
     #[serde(default = "default_size")]
     height: u32,
 
+    /// See [common options](module-level-options#common-options).
     #[serde(flatten)]
     pub common: Option<CommonConfig>,
 }