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/tab_switcher/src/tab_switcher.rs

@@ -25,6 +25,7 @@ use workspace::{
 
 const PANEL_WIDTH_REMS: f32 = 28.;
 
+/// Toggles the tab switcher interface.
 #[derive(PartialEq, Clone, Deserialize, JsonSchema, Default, Action)]
 #[action(namespace = tab_switcher)]
 #[serde(deny_unknown_fields)]
@@ -32,7 +33,15 @@ pub struct Toggle {
     #[serde(default)]
     pub select_last: bool,
 }
-actions!(tab_switcher, [CloseSelectedItem, ToggleAll]);
+actions!(
+    tab_switcher,
+    [
+        /// Closes the selected item in the tab switcher.
+        CloseSelectedItem,
+        /// Toggles between showing all tabs or just the current pane's tabs.
+        ToggleAll
+    ]
+);
 
 pub struct TabSwitcher {
     picker: Entity<Picker<TabSwitcherDelegate>>,