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/language_tools/src/syntax_tree_view.rs

@@ -15,7 +15,13 @@ use workspace::{
     item::{Item, ItemHandle},
 };
 
-actions!(dev, [OpenSyntaxTreeView]);
+actions!(
+    dev,
+    [
+        /// Opens the syntax tree view for the current file.
+        OpenSyntaxTreeView
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(|workspace: &mut Workspace, _, _| {