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/install_cli/src/install_cli.rs

@@ -8,7 +8,15 @@ use util::ResultExt;
 use workspace::notifications::{DetachAndPromptErr, NotificationId};
 use workspace::{Toast, Workspace};
 
-actions!(cli, [Install, RegisterZedScheme]);
+actions!(
+    cli,
+    [
+        /// Installs the Zed CLI tool to the system PATH.
+        Install,
+        /// Registers the zed:// URL scheme handler.
+        RegisterZedScheme
+    ]
+);
 
 async fn install_script(cx: &AsyncApp) -> Result<PathBuf> {
     let cli_path = cx.update(|cx| cx.path_for_auxiliary_executable("cli"))??;