Allow documentation of custom keymaps

[maxtremblay]

Hello, I don't know if it is already possible, but I found nothing. I just added a keymap to run the format command more easily.

[keys.normal]
"space" = { m = ":fmt" } # Format document

When I press space in a file and I wait for the helper menu, the command appears but the documentation is a bit weird as you can see in that screenshot.

It would be nice to have the option to add a custom label. In this case something like "Format document" would look much nicer than ":format []".

[the-mikedavis]

Duplicate of #1752