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/vim/src/normal/substitute.rs

@@ -7,7 +7,15 @@ use crate::{
     motion::{Motion, MotionKind},
 };
 
-actions!(vim, [Substitute, SubstituteLine]);
+actions!(
+    vim,
+    [
+        /// Substitutes characters in the current selection.
+        Substitute,
+        /// Substitutes the entire line.
+        SubstituteLine
+    ]
+);
 
 pub(crate) fn register(editor: &mut Editor, cx: &mut Context<Vim>) {
     Vim::action(editor, cx, |vim, _: &Substitute, window, cx| {