Update docs to describe use of custom commands without keybindings (#3846)

- **PR Description**

Update custom command documentation to describe how commands can be used
without keybindings (see [issue
3799](https://github.com/jesseduffield/lazygit/issues/3799#issuecomment-2297999051))

- **Please check if the PR fulfills these requirements**

* [ ] Cheatsheets are up-to-date (run `go generate ./...`)
* [ ] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [ ] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] If a new UserConfig entry was added, make sure it can be
hot-reloaded (see
[here](https://github.com/jesseduffield/lazygit/blob/master/docs/dev/Codebase_Guide.md#using-userconfig))
* [x] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

docs/Custom_Command_Keybindings.md

@@ -50,7 +50,7 @@ Custom command keybindings will appear alongside inbuilt keybindings when you vi
 For a given custom command, here are the allowed fields:
 | _field_ | _description_ | required |
 |-----------------|----------------------|-|
-| key | The key to trigger the command. Use a single letter or one of the values from [here](https://github.com/jesseduffield/lazygit/blob/master/docs/keybindings/Custom_Keybindings.md) | yes |
+| key | The key to trigger the command. Use a single letter or one of the values from [here](https://github.com/jesseduffield/lazygit/blob/master/docs/keybindings/Custom_Keybindings.md). Custom commands without a key specified can be triggered by selecting them from the keybindings (`?`) menu | no |
 | command | The command to run (using Go template syntax for placeholder values) | yes |
 | context | The context in which to listen for the key (see [below](#contexts)) | yes |
 | subprocess | Whether you want the command to run in a subprocess (e.g. if the command requires user input) | no |