docs: Add Rustdoc comments for info and mcp modules and grep plugin

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

PLAN.md

@@ -199,16 +199,16 @@ Private helpers (e.g., internal `fn` without `pub`) are not flagged, as they don
     - `new()` function: Partial.
     - Impl `MetaPlugin` methods: No docs.
 
-33. **src/modes/info.rs**
+33. **src/modes/info.rs** [DONE]
     - `mode_info()` function: Partial.
     - `ItemInfo` struct: No doc.
     - Helpers (`show_item`, `show_item_structured`): No docs.
 
-34. **src/modes/server/mcp/server.rs**
+34. **src/modes/server/mcp/server.rs** [DONE]
     - `KeepMcpServer` struct: No doc.
     - `new()` and `handle_request()` methods: No docs.
 
-35. **src/filter_plugin/grep.rs**
+35. **src/filter_plugin/grep.rs** [DONE]
     - `GrepFilter` struct: Partial.
     - `new()` function: Partial.
     - Impl `FilterPlugin` methods: No docs.