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/diagnostics/src/diagnostics.rs

@@ -48,7 +48,14 @@ use workspace::{
 
 actions!(
     diagnostics,
-    [Deploy, ToggleWarnings, ToggleDiagnosticsRefresh]
+    [
+        /// Opens the project diagnostics view.
+        Deploy,
+        /// Toggles the display of warning-level diagnostics.
+        ToggleWarnings,
+        /// Toggles automatic refresh of diagnostics.
+        ToggleDiagnosticsRefresh
+    ]
 );
 
 #[derive(Default)]