docs: Add missing docs for CommandInterceptResult fields (#39676)

Document the `string` and `positions` fields to resolve TODO comments.

Release Notes:

- N/A

Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>

crates/command_palette_hooks/src/command_palette_hooks.rs

@@ -97,11 +97,10 @@ impl CommandPaletteFilter {
 pub struct CommandInterceptResult {
     /// The action produced as a result of the interception.
     pub action: Box<dyn Action>,
-    // TODO: Document this field.
-    #[allow(missing_docs)]
+    /// The display string to show in the command palette for this result.
     pub string: String,
-    // TODO: Document this field.
-    #[allow(missing_docs)]
+    /// The character positions in the string that match the query.
+    /// Used for highlighting matched characters in the command palette UI.
     pub positions: Vec<usize>,
 }