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/lsp_tool.rs

@@ -19,7 +19,13 @@ use workspace::{StatusItemView, Workspace};
 
 use crate::lsp_log::GlobalLogStore;
 
-actions!(lsp_tool, [ToggleMenu]);
+actions!(
+    lsp_tool,
+    [
+        /// Toggles the language server tool menu.
+        ToggleMenu
+    ]
+);
 
 pub struct LspTool {
     state: Entity<PickerState>,