Document completion menu bindings (#10994)

* Update keymap.md

* Update keymap.md

* Update keymap.md

book/src/keymap.md

@@ -320,10 +320,14 @@ Displays documentation for item under cursor. Remapping currently not supported.
 
 Displays documentation for the selected completion item. Remapping currently not supported.
 
-| Key                         | Description    |
-| ----                        | -----------    |
-| `Shift-Tab`, `Ctrl-p`, `Up` | Previous entry |
-| `Tab`, `Ctrl-n`, `Down`     | Next entry     |
+| Key                         | Description                      |
+| ----                        | -----------                      |
+| `Shift-Tab`, `Ctrl-p`, `Up` | Previous entry                   |
+| `Tab`, `Ctrl-n`, `Down`     | Next entry                       |
+| `Enter`                     | Close menu and accept completion |
+| `Ctrl-c`                    | Close menu and reject completion |
+
+Any other keypresses result in the completion being accepted.
 
 ##### Signature-help Popup