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/command.rs

@@ -44,18 +44,21 @@ use crate::{
     visual::VisualDeleteLine,
 };
 
+/// Goes to the specified line number in the editor.
 #[derive(Clone, Debug, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 pub struct GoToLine {
     range: CommandRange,
 }
 
+/// Yanks (copies) text based on the specified range.
 #[derive(Clone, Debug, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 pub struct YankCommand {
     range: CommandRange,
 }
 
+/// Executes a command with the specified range.
 #[derive(Clone, Debug, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 pub struct WithRange {
@@ -64,6 +67,7 @@ pub struct WithRange {
     action: WrappedAction,
 }
 
+/// Executes a command with the specified count.
 #[derive(Clone, Debug, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 pub struct WithCount {
@@ -155,12 +159,14 @@ impl VimOption {
     }
 }
 
+/// Sets vim options and configuration values.
 #[derive(Clone, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 pub struct VimSet {
     options: Vec<VimOption>,
 }
 
+/// Saves the current file with optional save intent.
 #[derive(Clone, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 struct VimSave {
@@ -168,6 +174,7 @@ struct VimSave {
     pub filename: String,
 }
 
+/// Deletes the specified marks from the editor.
 #[derive(Clone, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 enum DeleteMarks {
@@ -177,8 +184,18 @@ enum DeleteMarks {
 
 actions!(
     vim,
-    [VisualCommand, CountCommand, ShellCommand, ArgumentRequired]
+    [
+        /// Executes a command in visual mode.
+        VisualCommand,
+        /// Executes a command with a count prefix.
+        CountCommand,
+        /// Executes a shell command.
+        ShellCommand,
+        /// Indicates that an argument is required for the command.
+        ArgumentRequired
+    ]
 );
+/// Opens the specified file for editing.
 #[derive(Clone, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 struct VimEdit {
@@ -1282,6 +1299,7 @@ fn generate_positions(string: &str, query: &str) -> Vec<usize> {
     positions
 }
 
+/// Applies a command to all lines matching a pattern.
 #[derive(Debug, PartialEq, Clone, Action)]
 #[action(namespace = vim, no_json, no_register)]
 pub(crate) struct OnMatchingLines {
@@ -1480,6 +1498,7 @@ impl OnMatchingLines {
     }
 }
 
+/// Executes a shell command and returns the output.
 #[derive(Clone, Debug, PartialEq, Action)]
 #[action(namespace = vim, no_json, no_register)]
 pub struct ShellExec {