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/activity_indicator/src/activity_indicator.rs

@@ -31,7 +31,13 @@ use workspace::{StatusItemView, Workspace, item::ItemHandle};
 
 const GIT_OPERATION_DELAY: Duration = Duration::from_millis(0);
 
-actions!(activity_indicator, [ShowErrorMessage]);
+actions!(
+    activity_indicator,
+    [
+        /// Displays error messages from language servers in the status bar.
+        ShowErrorMessage
+    ]
+);
 
 pub enum Event {
     ShowStatus {