editor: Add CollapseAllDiffHunks docs (#34124)

## Summary

This PR improves the documentation for diff hunk keyboard shortcuts to
make existing functionality more discoverable to users.

### Problem

Users were unaware that pressing **ESC** already collapses all expanded
diff hunks through the existing `Cancel` action. The functionality
exists but lacks discoverability:

- The `Cancel` action description was too generic: "Cancels the current
operation"
- No documentation existed for diff hunk keyboard shortcuts in the Git
docs
- Users would naturally expect a dedicated keybinding for collapsing
diff hunks

Release Notes:

- N/A

docs/src/git.md

@@ -112,10 +112,21 @@ by creating a [custom key bindings](key-bindings.md#custom-key-bindings) to the
 `editor::CopyPermalinkToLine` or `editor::OpenPermalinkToLine` actions
 or by simply right clicking and selecting `Copy Permalink` with line(s) selected in your editor.
 
+## Diff Hunk Keyboard Shortcuts
+
+When viewing files with changes, Zed displays diff hunks that can be expanded or collapsed for detailed review:
+
+- **Expand all diff hunks**: {#action editor::ExpandAllDiffHunks} ({#kb editor::ExpandAllDiffHunks})
+- **Collapse all diff hunks**: Press `Escape` (bound to {#action editor::Cancel})
+- **Toggle selected diff hunks**: {#action editor::ToggleSelectedDiffHunks} ({#kb editor::ToggleSelectedDiffHunks})
+- **Navigate between hunks**: {#action editor::GoToHunk} and {#action editor::GoToPreviousHunk}
+
+> **Tip:** The `Escape` key is the quickest way to collapse all expanded diff hunks and return to an overview of your changes.
+
 ## Action Reference
 
 | Action                                    | Keybinding                            |
-| -------------------------------------- | ---------------------------------- |
+| ----------------------------------------- | ------------------------------------- |
 | {#action git::Add}                        | {#kb git::Add}                        |
 | {#action git::StageAll}                   | {#kb git::StageAll}                   |
 | {#action git::UnstageAll}                 | {#kb git::UnstageAll}                 |
@@ -136,5 +147,7 @@ or by simply right clicking and selecting `Copy Permalink` with line(s) selected
 | {#action git::CheckoutBranch}             | {#kb git::CheckoutBranch}             |
 | {#action git::Blame}                      | {#kb git::Blame}                      |
 | {#action editor::ToggleGitBlameInline}    | {#kb editor::ToggleGitBlameInline}    |
+| {#action editor::ExpandAllDiffHunks}      | {#kb editor::ExpandAllDiffHunks}      |
+| {#action editor::ToggleSelectedDiffHunks} | {#kb editor::ToggleSelectedDiffHunks} |
 
 > Not all actions have default keybindings, but can be bound by [customizing your keymap](./key-bindings.md#user-keymaps).