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/terminal/src/terminal.rs

@@ -73,18 +73,36 @@ use crate::mappings::{colors::to_alac_rgb, keys::to_esc_str};
 actions!(
     terminal,
     [
+        /// Clears the terminal screen.
         Clear,
+        /// Copies selected text to the clipboard.
         Copy,
+        /// Pastes from the clipboard.
         Paste,
+        /// Shows the character palette for special characters.
         ShowCharacterPalette,
+        /// Searches for text in the terminal.
         SearchTest,
+        /// Scrolls up by one line.
         ScrollLineUp,
+        /// Scrolls down by one line.
         ScrollLineDown,
+        /// Scrolls up by one page.
         ScrollPageUp,
+        /// Scrolls down by one page.
         ScrollPageDown,
+        /// Scrolls up by half a page.
+        ScrollHalfPageUp,
+        /// Scrolls down by half a page.
+        ScrollHalfPageDown,
+        /// Scrolls to the top of the terminal buffer.
         ScrollToTop,
+        /// Scrolls to the bottom of the terminal buffer.
         ScrollToBottom,
+        /// Toggles vi mode in the terminal.
         ToggleViMode,
+        /// Selects all text in the terminal.
+        SelectAll,
     ]
 );