gpui: Store action documentation (#33809)

Closes #ISSUE

Adds a new `documentation` method to actions, that is extracted from doc
comments when using the `actions!` or derive macros.

Additionally, this PR adds doc comments to as many action definitions in
Zed as possible.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/agent_ui/src/language_model_selector.rs

@@ -18,6 +18,7 @@ use ui::{ListItem, ListItemSpacing, prelude::*};
 actions!(
     agent,
     [
+        /// Toggles the language model selector dropdown.
         #[action(deprecated_aliases = ["assistant::ToggleModelSelector", "assistant2::ToggleModelSelector"])]
         ToggleModelSelector
     ]