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/toolchain_selector/src/toolchain_selector.rs

@@ -15,7 +15,13 @@ use ui::{HighlightedLabel, ListItem, ListItemSpacing, prelude::*};
 use util::ResultExt;
 use workspace::{ModalView, Workspace};
 
-actions!(toolchain, [Select]);
+actions!(
+    toolchain,
+    [
+        /// Selects a toolchain for the current project.
+        Select
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(ToolchainSelector::register).detach();