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/git_ui/src/git_panel.rs

@@ -77,11 +77,17 @@ use zed_llm_client::CompletionIntent;
 actions!(
     git_panel,
     [
+        /// Closes the git panel.
         Close,
+        /// Toggles focus on the git panel.
         ToggleFocus,
+        /// Opens the git panel menu.
         OpenMenu,
+        /// Focuses on the commit message editor.
         FocusEditor,
+        /// Focuses on the changes list.
         FocusChanges,
+        /// Toggles automatic co-author suggestions.
         ToggleFillCoAuthors,
     ]
 );

crates/git_ui/src/git_ui.rs

@@ -31,7 +31,13 @@ pub mod project_diff;
 pub(crate) mod remote_output;
 pub mod repository_selector;
 
-actions!(git, [ResetOnboarding]);
+actions!(
+    git,
+    [
+        /// Resets the git onboarding state to show the tutorial again.
+        ResetOnboarding
+    ]
+);
 
 pub fn init(cx: &mut App) {
     GitPanelSettings::register(cx);

crates/git_ui/src/project_diff.rs

@@ -41,7 +41,15 @@ use workspace::{
     searchable::SearchableItemHandle,
 };
 
-actions!(git, [Diff, Add]);
+actions!(
+    git,
+    [
+        /// Shows the diff between the working directory and the index.
+        Diff,
+        /// Adds files to the git staging area.
+        Add
+    ]
+);
 
 pub struct ProjectDiff {
     project: Entity<Project>,