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/collab_ui/src/collab_panel/channel_modal.rs

@@ -17,9 +17,13 @@ use workspace::{ModalView, notifications::DetachAndPromptErr};
 actions!(
     channel_modal,
     [
+        /// Selects the next control in the channel modal.
         SelectNextControl,
+        /// Toggles between invite members and manage members mode.
         ToggleMode,
+        /// Toggles admin status for the selected member.
         ToggleMemberAdmin,
+        /// Removes the selected member from the channel.
         RemoveMember
     ]
 );