Update docs to describe use of custom commands without keybindings

[a-jentleman]

• PR Description

Update custom command documentation to describe how commands can be used without keybindings (see issue 3799)

• Please check if the PR fulfills these requirements

• [ ] Cheatsheets are up-to-date (run go generate ./...)
• [ ] Code has been formatted (see here)
• [ ] Tests have been added/updated (see here for the integration test guide)
• [ ] Text is internationalised (see here)
• [ ] If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• [x] Docs have been updated if necessary
• [x] You've read through your own file changes for silly mistakes etc